It's kinda shitty that it isn't possible to wait till a kid is at least a few years old, and capable of making at least a somewhat-informed decision about circumcision, without it already being too late to avoid certain medical complications if he wants to do it.
My thinking is, it seems like better engineering to get rid of moving parts if you can do so without sacrificing important functionality or aesthetics. Still, as a matter of principle, I don't much like the idea of forcing a medical procedure, particularly an amputation, upon those who clearly don't consent to what is happening. But I guess it would be the same with any other painful medical procedure that the parents thought was in the child's best interests. When it's possible to delay the procedure until he has a chance to make his own decision, especially if the procedure is potentially controversial, then I think it should be delayed.
My rule of thumb is that normally, you should quit doing what a child (or other person) clearly objects to (whether the language available to them for voicing such displeasure is English, or crying, or whatever; either way, lack of consent is pretty plainly evident by body language, etc.). I think if a 3-year-old exhibiting signs of ability to make a reasonably intelligent decision were to decline a lifesaving medical treatment, I would respect his decision; it would be different with an infant.
I think that to protect life, liberty, and property, the bar for declaring a child competent should be set very low, kind of like how in court, all a defendant has to do, to be declared mentally competent to stand trial, is "understand the nature and consequences of the proceedings against him or to assist properly in his defense". (See 18 U.S.C. § 4241)
Sex is a bit different; since a child can understand even as an infant whether it's pleasurable or displeasurable to him, and sexual acts that don't cause physical harm are unlikely to cause psychological trauma in the short time between when it's initiated and when the child has an opportunity to object, he's capable of making sufficiently informed decisions at any age as to whether he wants to engage in that activity, and adults should respect those decisions. Sexual acts with potential to cause physical harm (e.g. sexually transmitted infections) require a higher level of informed consent.
So, to put it in the form of a decision tree: If (1) the proposed action is unlikely to cause what might be reasonably construed as physical or psychological harm in the interval between when it is initiated and when the child has an opportunity to object, and (2) the child adequately understands all issues relevant to making an informed decision on the matter in a way that will protect his interests, then it is okay to do it and see what his reaction is; if he's okay with it, then it can continue. Example: adult-child sex.
If refraining from engaging in the proposed action is likely to cause what might be reasonably construed as physical or psychological harm in the interval between when it is initiated and when the child reaches a point of being able to adequately understands all issues relevant to making an informed decision on the matter in a way that will protect his interests, then the parents can take action that the evidence at hand suggests is reasonable for protecting his interests. Example: circumcision