please refund my money my child is purchased the item and it isnot work

On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name].
2. Tap Family Sharing.
3. Tap your family member's name.
4. Tap Ask to Buy.

Request and make purchases with Ask to Buy - Apple Support

As far as my knowledge goes there are no refunds for accidental or unintentional purchases, however...

Unknown Charge

• Curious about a Charge? --> If you see an 'itunes.com/bill' or an unfamiliar charge on your statement
• If you have an issue with an app on App Store or billing --> Request a refund for an App Store or iTunes Store
• You can also request a refund for a recent renewal of an in-app subscription.purchase 

Phishing

• Beware of --> Phishing & Other Suspicious Emails 
• Know how to --> Identify legitimate emails from the App Store or iTunes Store 
• Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams —> Use these tips to avoid scams and learn what to do if you think your Apple ID has been compromised.

Start here: https://support.apple.com/en-us/HT204084