Contacting Apple using the links already provided can be done either with a direct call or by Message. Refunds are not guaranteed and no one here will be able to predict the resolution, but a follow up on your experience with support may help others who find themselves in the same situation.
It is important to note that Apple provides tools such as Screen Time restrictions to prevent this from happening. If a child knows you Apple ID/Password and Passcode, you are giving them permission to use your device and that includes purchases. To prevent a purchase in the App Store, set up restrictions with a different Passcode by following this support article:
Use Screen Time to prevent purchasing - Apple Support
If you have not done so already, change your Passcode and Apple ID Password:
Set a passcode on iPad - Apple Support
Change your Apple ID - Apple Support