My child keeps turning off downtime on their phone. How do I prevent this in family sharing?

Did you lock the feature in Screen Time? Did you disable all the workarounds kids can try to get around this such as changing time or time zone?

Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support

Change Downtime preferences in Screen Time on Mac - Apple Support

Contact Apple. Click here --> Choose your country or region - Official Apple Support 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 --> Contact Apple for support and service - Apple Support

I believe you mean Screen Time? --> Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support

There is no official Apple documentation about the issue you are having but if you browse the web in general you will find reference to the issue. You may need to increase the number of things you block. For example, one common way to get around this was for a child to change the time or time zone on their phone, so you would need to expand blocking to include those features.