Boost Mobile has introduced a really cheap plan, $10/month for 2GB Data and Unlimited Talk & Text. I've looked for gotchas on this plan but can't find any. They'll send you a SIM for FREE and it appears that once you get this plan it doesn't expire. My kid is in Wi-Fi coverage all day so doesn't mind the 2GB limit and at this price he can pay for it by himself.
Boost Mobile needs to be aggressive right now as they introduce their own 5G network, but for the foreseeable future it looks like you'll be roaming more on AT&T or T-Mobile until you switch to one of Boost's new 5G phones that automatically switch on and off their own 5G network. This is a limited-time offer so decide soon.
If you need a little more Data, Boost has a $15 plan for 5GB, and also an Unlimited Data plan for $25/month.