Look into the BlackBerry Q5, Q10, or Q20 (Blackberry Classic).
Technically speaking not a dumbphone but it has a very strong inbuilt parental control feature to block internet, whatsapp is not available anymore, could be email still works but can be blocked. Most apps don’t work anymore.
BlackBerry has ended support for these phones so do some research before buying that the phone will still work with your carrier etc. Best resource for all blackberry advice is crackberry.com
Be aware they are hard to find, but try eBay.
I strongly believe there is no better dumbphone on the market then (the pre-android) BlackBerry’s.