Can a child under 13 access his iCloud if he is part of my family sharing?

Yes. Create an Apple ID for your child says, with regard to creating a child's Apple ID, that a child, "can use Family Sharing, Messages, the App Store and other Apple services."

Your question suggests that you have a child who uses an individual iCloud storage plan which you suspect might be lost in the migration to a family plan. This won't happen. In accepting an invitation to join a family group and a family storage plan the individual storage plan is converted to the family storage and no data is lost.