Its the childs body, its the childs choice when they grow to make that choice as an adult, or at least when they're old enough to be reasonable.
The problem with circumcision is so many people do it to their kids, the uncut kid is the minority. When they get into any school setting (even private, gov. school haters) they get involved in locker room stuff, or standing at the urinal, they see each others penises. In the ignorance that is young boys, they eventually run across the uncut kid who thinks he is normal, then they laugh at him. This is psychologically damaging to the self perception and sexuality of the uncut kid.
If all kids were left uncut until they were more mature, this wouldn't happen. And most girls will have by then been exposed to uncut penises, so they wouldn't think it was weird, which in turn would further reduce the demand for cutting in normal society (aside from religious reasons, I mean).