Kids will heal much, much quicker than adults, but be patient to make sure it's fully healed, i.e. calcified and hard. Also, what was the break like? A hairline fracture, and complete breakage with a gap between the bones? How many breaks?
I broke mine in 2 places, with some gaps, and it was 4 months before I felt it was strong enough to go riding again. (I'm 38 - also broke it on the same side I broke at age 18).
The thing is, even though it feels fine, you can re-break it easily until it's completely calcified. I've heard many stores of guys who have re-broken their collabone, at 4-8 weeks out because they felt fine and couldn't wait. One re-broke it shooting a basketball, the other swatting a fly suddendly. Then you are essentially back to square one.
Your doc and x-rays know/reveal the real story, so go with those, not an anxious 6-year-old.
Good luck!