Halacha forbids eating and drinking before Kiddush on Friday night on Shabbat morning. However, the Magen Abraham (Rabbi Abraham Gombiner, Poland, 1637-1683) and Mishna Berura (Rabbi Yisrael Meir Kagan, 1839-1933) write that this prohibition applies only to adults, and was not stated in regard to children. If a child wishes to eat or drink before Kiddush, he may be allowed to do so. They may even be given sweets and treats that are not necessary for their growth. Parents should not act stringently in this regard, and should permit their children to eat and drink as usual before Kiddush. Very often, when children come to the Shabbat table they immediately want to take something they see on the table and eat. Parents should allow the children to eat and should not make them wait until after Kiddush.
The exception to this rule is the case of a child approaching the age of Bar/Bat Misva, who should be trained in Halachic observance. For example, a boy who is already twelve and a half years old, and is thus within half a year of Misva obligation, should be trained not to eat until after Kiddush. Otherwise, parents should not stop their children from eating before Kiddush, neither on Friday night nor on Shabbat morning.
Summary: Although one may not eat or drink before Kiddush on Friday night or Shabbat morning, children who wish to eat or drink before Kiddush should be allowed to do so, unless they are within a few months or so of Bar/Bat Misva age.
