how do I turn off share across devices for my child on my phone?

Hello laurenkate845.

Thanks for reaching out to Apple Support Communities. We understand that you would like to stop sharing Family purchases. We will be glad to assist. Please see the step below:

On your iPhone, iPad, or iPod touch:

1. Tap Settings > [your name] > Family Sharing. On iOS 10.2 or earlier, tap Settings > iCloud > Family. 
2. Under Shared Features, tap Purchase Sharing.
3. Make sure that Share My Purchases is turned off, and that your account information is correct.

For more information on Family purchases see the following: Family purchases and payments

Cheers.