The main feature I want for my kids is an explicit allow list to keep them from gettin spam or talking to randoms.

The manual I found doesn't seem to have this feature. Also the advertising page has "Cloud Apps for news, weather, and more" :( https://www.hmd.com/en_int/nokia-3210?sku=1GF025CPD4L02

Yes you should check KaiOS phones for that.

Now the problem is that they're not "dumb" enough as they have a web browser, youtube etc, but that can be considered fine because the experience for those is so bad that it's unlikely to get addicting.