How to edit my childs birthdate in family sharing

If it's a child account (i.e. under 13) you can't, though you could try contacting iTunes Support and see if they can/will change it for you : http://www.apple.com/support/itunes/ww or https://www.apple.com/emea/support/itunes/contact.html

If it's not a child account log into the account : Update the birth date associated with your Apple ID - Apple Support

On her iOS device:

1. Go to Settings > tap on her name.
2. Tap Name, Phone Numbers, Email.
3. Tap on Birthday (You might need to answer your security questions there).
4. Update her birth date, then tap Done.