iPhones have quite a few inbuilt things you can do to set them up for a child - I googled it and followed the steps to set it up. Mine have app limits, downtime, content restrictions, only certain apps allowed etc and you can also set whether they are allowed to download new apps or need the ask for permission first, make in app purchases etc. I have to admit it took me quite a while to work through it all! If you have an iPhone too you can monitor usage and controls from your phone.
The other thing I’d look into is Life360 - allows you to see where they are, get notifications when they arrive at school etc - quite reassuring when they’re first starting to travel by themselves and be a bit more independent.
Regarding contracts, mine are tied to my EE contact. They have some very cheap SIM only options and I can gift data to them that way rather than paying more on their contracts. EE also have some safety features built in to protect children too.