If you already have a pixel: Maybe look into Graphene OS. With that, you can completely decouple your child's data from google.

Google could add a feature to their OS to upload all pictures to their servers, but that'd likely violate laws and is easy to detect (unusual network traffic).

Thus it is extremely unlikely they upload your pictures to their servers if you disabled their cloud backup feature.

What's more likely is that they hash your pictures locally, and upload the hash to their servers, like some government proposals want to make them do (to find known CSAM, they say). Or they could track metadata of the pictures (location, date/time). This would be hard to detect and some of the info they likely already have through other features in the OS. Whether they do those things or not I don't know.

This data wouldn't reveal the content of the pictures, so nobody can see your images.

open camera ☞ https://f-droid.org/packages/net.sourceforge.opencamera/ or https://sourceforge.net/p/opencamera/code

aves libre ☞ https://f-droid.org/packages/deckers.thibault.aves.libre/ or https://github.com/deckerst/aves

fossify gallery ☞ https://f-droid.org/packages/org.fossify.gallery/ or https://github.com/FossifyOrg/Gallery

and you can uninstall google's camera and gallery :)

with open camera you choose what metadata to include. My photographs only have date & time.

aves has built in vaults that prevent other apps or users from accessing your images

Safest measure, from what I understand, is to completely replace the system for a vanilla one - that is, without any software from Google or related - and then install alternatives unrelated to the datahoarding companies.

I guess you could read the TOS of the OS... But I'd think that they wouldn't collect data on photos unless uploaded to Google photos