If you want to keep everything on the iPod, simply do not restore it. Leave it as is, and just sign out of the current Apple ID, and sign in with the new child account you made over family sharing.
Settings ➜ Apple ID ➜ (scroll to the bottom) ➜ Sign Out.
Settings ➜ Apple ID ➜ Media & Purchases ➜ Sign Out
Settings ➜ Messages ➜ Send & Receive ➜ Apple ID ➜ Sign Out
Settings ➜ Facetime ➜ You can be reached by facetime at ➜ Apple ID ➜ Sign Out.
If you want to save it, you connect the iPod to a computer and back it up there, or transfer any images and videos from the Photos App to the computer if you need to.
click here ➜ Transfer files between iPod touch and your computer - Apple Support
and here ➜ Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support
or here ➜ Back up iPod touch - Apple Support