How can I delete a child I made on mistake?

Hi yanirab,

Thanks for posting in Apple Support Communities! Although you're unable to remove accounts for children under 13 from your family group, you can delete the account. Follow the guidance below to do so from the article How to delete your Apple ID account:

Sign in with your Apple ID at privacy.apple.com on a Mac, PC, iPhone, or iPad, and select “Request to delete your account”.

When you request account deletion, you'll receive a unique alphanumeric access code. Make sure that you keep this access code in a safe place. For a short period of time while your request is being processed, you can contact Apple Support, provide your access code, and we’ll help you cancel your request so you can resume using your Apple ID. 

When the account is deleted, it will be removed from your Family Sharing group. More about managing children with Family Sharing can be found here: Move a child to another group using Family Sharing.

Hope that helps!