Why place physical limits when virtual limits will do
One of the issue with feature phones is that nowadays you can find it hard to navigate life without a app. Banking is the worst for this. Yes, you can still do telephone banking or even in person banking, if you can wait 40 minutes in a queue or drive 10 miles to the nearest branch that hasnt closed down. Event and airline tickets are almost as bad, mostly now needing a download, which you can print out at home or pay a fee to have printed and sent to you. With a feature phone you have no choice but to put up with the friction this brings to life.
I think the better answer for children may be to give them the smart phone that they desire but functionally limit it. This can certainly be done with both Android and iOS. On iOS, which I am familiar with, you can have total control over what apps are available and usable on the phone and in many cases also control what features of an app can be used. This is all done in in Settings->Accessibility->Assistive Access. Once set up you can limit the user to whatever apps you approve of, eg phone, messages, maps but not (say) social media or the app store.