how to update game on kids iphone that is conected into family sharing on my iphone, as underage.

The child should be able to update it:

Approve what kids buy with Ask to Buy - https://support.apple.com/HT201089 -

"If a child redownloads a purchase, downloads a shared purchase, installs an update, or uses a redemption code, the family organizer won’t receive a request."

Do you really mean the account is locked?

Read the support article "If your Apple ID is locked or disabled"; it has solutions for two frequently encountered reasons for an account to be disabled/locked. Click here --> https://support.apple.com/HT204106

If you still have questions or problems:

Contact Apple. Click here --> https://support.apple.com/choose-country-region Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> https://support.apple.com/HT201232