May a child below Bar Mitzvah eat before Davening Shacharis [i.e. Hamotzi breakfast]:
A. Background-The general prohibition of eating before prayer:
The sages prohibited one from eating or drinking starting from the time of Alos Hashachar until after one Davens Shemoneh Esrei [of Shacharis]. This is in addition to the prohibition to begin a meal starting from 30 minutes prior to Alos. Despite this law, it is permitted to eat foods and have drinks prior to Davening for medical purposes. However, it remains forbidden to eat or drink prior to Davening for the purpose of satiating one’s thirst or hunger, unless one is not able to have proper concentration in his prayer until he eats or drinks. This allowance applies even in today’s times when in general people do not have much proper intent during prayer, nevertheless, if he chooses to eat and drink for this purpose of strengthening his concentration, then he may do so.
B. The law:
Although children who are above the age of Chinuch are obligated to be educated to pray Shemoneh Esrei both by evening and morning, nonetheless, it is permitted to give them to eat [without restriction] prior to them Davening Shacharis [until they reach the age of Bar/Bas Mitzvah], and it is even forbidden to suppress them, as explained regarding the allowance to give them to eat before Kiddush. [Accordingly, there is no restriction to give children before the age of Bar Mitzvah to eat breakfast before Shacharis, and they may eat Mezonos, and even wash on bread for breakfast, or eat snacks and ice cream as they wish. Despite the above ruling, as the child becomes older, such as from the age of 12 and onwards for a boy, one may encourage the child to not eat unnecessary foods before Davening by stating to him that it is proper for him to abstain on his own until after Davening, even though if he wants to eat it, one may not stop him from doing so as stated above. Once the child turns Bar Mitzvah, he may only eat foods necessary for his health and extra concentration in prayer, as stated above.]
Prior to the age of Bar/Bas Mitzvah, a child may eat prior to Davening without restriction, and may therefore eat a meal with bread and eat treats and the like and it is forbidden for a parent to oppress his child with these restrictions. Nonetheless, from age 12 and onwards for a boy, it is proper for parents to begin encouraging their child to resolve on his own not to eat unnecessary foods before Shacharis. Foods that are needed to be eaten for health purposes, and extra concentration of prayer, may be eaten even after bar Mitzvah.
 See Admur 89:5-6 and 70:5; Michaber 89:3; Tur 89:3; Rebbe Eliezer Ben Yaakov in Brachos 10b
 The reason: The Sages supported this prohibition on the verse [and negative command] of “Lo Sochlu Al Hadam:do not eat on the blood” [Vayikra 19:26], which [the Sages] expounded to mean that one is not to eat until he first prays on behalf of his own blood, and that whoever eats and drinks and only afterwards prays, on him the verse [Melachim 1 14:9] states, “Veosi Hashlachta Achar Goyecha:You have thrown Me behind your Goyecha [lit. body].” Now, one should not read this word as Goyecha which would imply to include all of one’s bodily needs, but rather it should be read as Geiecha, which comes from the word of arrogance-Gaava. Accordingly, Hashem is lamenting in this verse that only after one first ate [and pampered himself] with food and drink and became haughty, did he accept the yoke of Heaven. [Admur ibid] This is in addition to the general prohibition of eating a meal prior to fulfilling a command, such as Shema and Shemoneh Esrei. [Admur 70:5 and Kuntrus Achron 70:2; Rama 581:2]
 See Admur 70:5; Kuntrus Achron 70:2; Although snacking is still allowed until Alos. Beginning from Alos, it becomes Rabbinically forbidden for one to eat any food, even a mere snack, prior to Davening Shemoneh Esrei.
 The reason: As foods which are eaten as medicine are not considered to contain any haughtiness. [Admur ibid]
 It is however forbidden for one to eat simply because he is hungry or thirsty, without having intent to do so to strengthen his concentration of prayer, as stated above.
 Admur 106:3; M”A 106:3 and 269:3; Machatzis Hashekel on M”A ibid; M”B 106:5; Piskeiy Teshuvos 89:19; See also Admur 269:3 and 343:7 regarding Kiddush in Shul;; 471:10 [regarding eating before Kiddush and Erev Pesach]; 472:23 [regarding eating before start of seder at night]; M”A 106:3; 269:1; Tzemach Tzedek O.C. Safek Limi 4 M”B 269:1“It is permitted to feed children food on Shabbos morning prior to Kiddush and it is forbidden to oppress him”
Other opinions: Some Poskim rule that children who have reached the age of Chinuch should be educated not to eat before Davening. [Elya Raba 106:2; Kaf Hachaim 106:11]
 Regarding the age, see Admur 343:3 that the age of education for positive commands depends on the sharpness and knowledge of that individual child in each particular Mitzva, whether a Biblical or Rabbinical command. For example, a child who understands the concept of Shabbos is obligated to hear Kiddush and Havdala. If the son knows how to garb himself in a Tallis, then the father is obligated to educate the child wearing Tzitizs. See P”M 106 A”A 3 who writes the practical age for Chinuch for Davening is between age 6-7; See also M”B 70:6; Piskeiy Teshuvos 106:2 footnote 35
 Maariv. [Implication of Admur ibid; Salmas Chaim 36; Piskeiy Teshuvos 106:2]
 Admur ibid; Michaber 106:2; Mishneh Brachos 20a
 See Admur 269:3; 343:7; M”A 269:3
The reason: As the [Sages] only prohibited feeding [Rabbinically] forbidden foods [to a child] when the food is forbidden in it of itself, such as a piece of Treifa [Niveila] and the like. However, if the food is Kosher in it of itself and it is just forbidden to be eaten within a certain time slot, then it is permitted to be fed to a child. A proof for this is the ruling regarding feeding a child on Yom Kippur in which case one may do so even if there is no danger involved for the child to fast. [Admur 269:3; M”A 269:1; Yevamos 114b; Machatzis Hashekel on M”A 106:3]
 See Yabia Omer 4:12 that a healthy child should be educated in this from age 12
 See Admur 343:6 “Even though the child can wait to hear Kiddush in the place of a meal, in the house of his father which is educating him in this.” One must establish that this refers to the father encouraging the child not to eat until Kiddush, as to force the child not to eat is forbidden as stated above, and thus from here we can conclude that one may educate the child through encouragement [Perhaps however this is referring to educating the child not to eat in a case that he does not feel a need or will to eat or drink before Kiddush and is being given to eat for other purposes-Vetzaruch Iyun]; See also Admur 343:7 “Therefore it is permitted to feed him before Kiddush if he wants to eat” which implies the child does not have to be educated to eat if he does not want to [See also Tzemach Tzedek O.C. Lo Noda ,Lemi 4 who raises contradictions of this sort in Admur] So also rules: Or Letziyon 2:47-6; Piskeiy Teshuvos 269:2; See Likkutei Dibburim, vol. 4, p. 1418 that Rabbi Yosef Yitzchak of Lubavitch related: When I was seven years old, my father [Rabbi Shalom Dov Ber of Lubavitch] said to me on the eve of Yom Kippur: “In the evening and all night it is forbidden for you to eat. Tomorrow in the morning until noon you do not need to eat. From then on it depends on your will.” My father explained to me the gravity of eating on Yom Kippur, and concluded: “If you want to eat, do not ask anybody, only come to me. I have prepared food, water, and juice for you. If I am in the middle of the Amida prayer, wait for me; just be careful not to ask for food from anyone else.” That year I completed the fast for the first time in my life, and the year after that it was already an easy matter.