How can I fixed my kid age in her Apple ID account?

How old is your child?

If they are younger than 13 (in the US, it varies across the world) you cannot.

Update the date of birth associated with your Apple ID – Apple Support (UK)

As the page above states once her age is set to be below 13 it cannot be adjusted.

However, you should be able to create a new AppleID with the proper year of birth and have her use that as long as she hasn’t yet made any purchases with it.