10 yr old has accessory navicular fr. Had boots for 3 wks. Walked/played a lot, didn't heal and is in cast for 3 weeks. Is 3 weeks enough to heal now?

Healing fractures. If your child is compliant with recommendations about rest and limitations on activity, 3 weeks should be enough. If they continue with normal activity it will prolong healing even longer (that's why they had to have a cast).