Inspired by this I hooked up an old rotary phone to an ATA and got a number from a low cost SIP provider. Now my 5 year old can call his friends and family on his own. He loves when it rings and when he can call his pals to make his own plans.
It lacks the features and network effects of the tin can system but is still pretty fun.
You can't restrict callers as easily on a landline phone. You can block individual numbers, but most landline providers won't let you block by default. I can only imagine the support calls Comcast would get from people following some "never get spam!" guide but then not being able to get calls from their doctor or the plumber.
If you're handy with tech like most folks on this forum, you can do all this yourself through a voip provider for significantly less per month, but you don't get a nice app interface to configure and you'll have to scrounge and hack to get cute hardware for your kid.
It lacks the features and network effects of the tin can system but is still pretty fun.