You have a few options on what you can do to limit your child’s usage of a device.

NextDNS has some interesting features for parental controls, as well as some options to try and prevent bypassing it (i have yet to try it though). On iOS it can be installed as a device profile and you can prevent them from being uninstalled, I’m not sure how to go about this on android but I’m sure there’s a way of limiting the child’s access to changing the settings.

Otherwise the built in parental control features on Android and iOS seem to be very useful as well, since you can limit what your child can install, set screen time limits, etc. I am pretty sure you’d want a more recent version of Android however to get all of the features here which may not be possible depending on the budget.

If you just want the ability to call / text then perhaps a dumb phone is a good first step? But a smart phone may be more useful if they’re older.