Resetting iPad to factory settings

Welcome to the community.

Simple - follow this guide:

https://support.apple.com/en-gb/HT201351

Resetting your iPad will not affect your iPhone - but just in case, ensure you have a taken an iCloud (or PC/Mac) backup of your iPad before proceeding with the reset process. Then if anything unexpected happens, you can simply restore the iPad to how it was before you started.

I hope you find this helpful in resolving your question.

Just to clarify this a little bit further, make sure you backup the old iPad first so that you can then transfer the contents to the new device. There are several different ways that you can transfer the contents to the new iPad. You can read about your choices here.

Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

There is also a link in the article that takes you to a support article about backing up, if you don’t know how to do it. After you have transferred the contents to the new iPad, you can erase the old iPad before giving it to your granddaughter.

I would wait to erase the device until the new device has been successfully set up.