Why do u think Ender's parents must let him go to battle school?

Ender's parents didn't really have a choice about whether he would attend battle school or not. When they were given permission to have a third child, that permission was based upon the promise of their first two children. Ender was allowed to be conceived for the service he might be suited for later in life. Quite frankly, the possiblity of having to give him up was all a part of the bargain, which allowed them to have a "third".