I am very curious what other's input will be, as this is an important question!

I don't know how old they are, but kids these days are usually pretty tech savvy so it will be hard. You can get an app locker app to password protect certain apps like app stores and web browsers, but even then if they find out they can uninstall the app locker to disable the protections, then you're back to square one. Brave Browser has content filtering which you can enable to block malicious websites, foreign sites, p*rn sites, and specific domains you choose to block. Same thing though, this can be disabled if they know where to look. An advanced option would be to set up a proxy on another computer that the phone links to, and that computer acts as a firewall, so they'd have to have access to that computer to unblock certain things. On another note, as someone who grew up with protective parents, it only creates curiosity. My personal opinion is to either not give them a phone or teach them good digital habits based on trust. The ladder really resonates with a child in a positive way :)

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.