I got my older kids apple watches set up in family/parent/whatever mode for this. They have cell modems (requirement for family setup, they're $5/mo each on my t-mobile plan), can send and receive texts, can send and receive phone calls.

If you don't want to use apple watches because of their ridiculously poor battery life (fair criticism; I have trouble remembering to charge mine regularly, so getting the kids to do so regularly isn't easy), you can find used/old iphones lying around for pretty cheap, and Apple Configurator lets you put a pretty tight policy on them. You can set it up so that the profile can’t be removed without a password. I think you can even set it up so it has wifi credentials preloaded and so that the user can’t add other wifi networks, and that you can preload contacts and prevent the user from calling/texting numbers that aren’t in the contacts (I don't think you can prevent them from answering calls / receiving texts from numbers not in their contacts, though).

I have an old iphone with such a profile that only has the Phone app, Messages app, and FaceTime. It doesn’t have the app store and as far as I know there’s no way to install or sideload apps.