What you could do is use an apk downloader to remove the file from Google
Play and then side load it on the Replicant device. Now, depending on the
phone the app might not work due to the many limitations of Replicant, but
you won't know until you try it.
An example:
https://lifehacker.com/apk-downloads-lets-you-pull-apk-files-directly-from-goo-1456775931
In order to keep those sideloaded apps updated see:
https://android.gadgethacks.com/how-to/get-easy-updates-sideloaded-android-apps-0174291/
The more straight forward solution is to work with your child to teach them
about the dangers of software and why software freedom is important. Kids are
a lot smarter than people give them credit for. The other point is if free
software is a value of yours that you want to instill in them you shouldn't
buy them compromised systems in the first place so that don't grow attached
to their stickiness. That might be unpopular with your kid, but parenting
isn't a popularity contest as I am sure you already know.
