Editing my child's Apple ID

On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name].
2. Tap Name, Phone Numbers, Email. You might need to enter your Apple ID password.
3. Tap Change Birthday or Birthday.
4. Update your birth date, then tap Done. If you're changing a birth date for a child, learn what to do.

Update the birth date associated with your Apple ID

It would be better if you can please re-post with a screenshot, in order to understand the phenomenon better. Images/pictures speak louder than words. 

Please understand this is a public forum: While attaching a Screenshot, do not provide any personal credentials, such as IP address, Card detail, email id, apple id, IMEI number, Serial Number, Phone number, Order ID, if you are sharing a map ensure your location is not identifiable, etc.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.