Of course we are not talking about state-controlled penis inspections, but during routine examinations, a doctor would of course notice that a child has been circumcised and ask the parents if there was a medical indication for that (which it never is in the age when religious circumcisions are performed btw).

Forced circumcision without medical indication is a violation of a child's bodily integrity and a free society cannot tolerate such behavior, because doing so would bring us down an even worse slippery slope. What if parents beat their children black and blue, or abuse them in other ways? At which point do you draw the line, where we need to protect children from their parents?

In a free society, any such violations of the non-aggression principle must be sanctioned appropriately.