C.C.
Every community is different, but, in general, I think many middle-school aged children walk to school if they live close enough to do so. So, in general, I think that crossing the street safely is a skill that should be learned by age 11.
BUT, for people of any age, every town has those certain intersections that are just awful, blind spots etc. If this intersection you describe is like that, then I think it is worth thinking about whether there is another route the boys could take to walk to the park - even if it means "going the long way", that might be preferable to dealing with the bad intersection and also preferable to trying to forbid the park walk entirely.