The Church itself really has nothing to say on this, that is insofar as going beyond St. Paul's general admonition about eating during liturgies (1 Cor. 11:22). That is because this is mainly an issue that has been handled by society. In Western society, it is inappropriate for anyone, no matter their age, to eat in the sanctuary during a worship service (assuming, of course, that we are speaking of ordinary eating and drinking, and not ritual meals like the Eucharist). If you are not convinced by the general social norm that frowns on snacking in a church merely as a matter of decorum, perhaps you might consider the concerns of modern public health specialists, who point to the health dangers of giving kids food solely as a means to occupy them, to amuse them, or to relieve their boredom.