how can i remove pending purchases?

You can set the App Store to require a password for purchases, or use Family sharing and Ask to Buy to prevent a child from simply purchasing things unsupervised.

See here: link-> Approve what kids buy with Ask to Buy - Apple Support

You cannot. Pending Purchases must be completed as stated in the terms of service of the iTunes & App Stores. Once completed you can request a refund for them.

Pending purchases indicate a purchase that was unable too be charged successfully to your payment method or Apple ID balance, as such you are responsible for that purchase, and must allow it to successfully complete before anything else can be done.

Once it has been successfully charged, you can go to https://reportaproblem.apple.com and request a refund.