How do I cancel a child's accidental in-app purchase on iPhone?

You can follow the directions below to request a refund. But please keep in mind, refunds are not guaranteed.

Copied from here: Request a refund for apps or content that you bought from Apple - Apple Support

How to request a refund

1. Sign in to reportaproblem.apple.com.
2. Tap or click "I'd like to," then choose "Request a refund."
3. 
   
4. 
5. Choose the reason why you want a refund, then choose Next.
6. Choose the app, subscription, or other item, then choose Submit. If you were charged for a subscription that you no longer want, you can also cancel the subscription.

Where did you see that message? - Not enough money? If you follow the directions I provided, the response will be Accepted or Rejected.

You may want to put your payment card in a more secure place where your child cannot get to it.