The Gemora tells us that if we promise something to a child we must deliver, based on the pasuk “Limdu leshonam dabeir sheker,” since by not keeping our promise we would be teaching the child the concept of falsehood. This holds true even if according to halachah it would be permissible to renege on our promise; nonetheless, because of the educational component, we should not do so. If we promise a child something in return for an activity performed, it might be considered like an employer-employee relationship, and we are obligated to fulfill our commitment specifically for that reason. If we promised and actually gave a gift to a child who is on the level of maturity which halachah refers to as “Tzror v’zorko, egoz venotlo”—if they get a pebble, they will throw it away, but if they get a nut, they will know to take it and keep it—then any gift they receive becomes theirs and we are not permitted to take it back. If a child is younger than that age, then min ha’Torah they cannot legally acquire anything; nevertheless, according to one opinion mi’derabbonon, rabbinically, we may not take back the gift.