Maybe they could set up some geo-fencing so at least when the child leaves the area served by wi-fi, it would send them a notification.
But yeah, it will be tricky without having a data plan of some kind. They could at least pare down the data plan so only this feature is using data.
There is also SPOT from Globalstar that is a $20/month subscription.
https://www.findmespot.com/en/index.php?cid=130