If it's a child's decision to get his pee-pee cut, then it can also be argued we as parents should give the child the decision making control for everything in his / her life.
Let's let them chose what they want to eat for dinner; let's let the kiddies choose what time we go to bed; let's let them entirely choose who and what they do in their free time, whether it be drugs, sex, booze, video games, painting, karate, farming, shitting; let's let them choose to get vaccinated or not; let's let them choose to take their own life if they see it a proper decision.
Point being, children often don't know how to make proper decisions; they aren't mentally capable of grasping the importance of many concepts. As such, giving a child the right to choose whether or not to be castrated is like giving an elephant caviar: entirely pointless. Of course, you could always argue that they could "wait until they're, like 18, to have it done". Well, then that's another story.
Now I have to go to work.