We have no clue.
I'm on a very cheap pay monthly with a 2 year old phone and dad still has a PAYG (yes they do still exist!!)
DC has absolutely zero interest in having a phone but it's becoming a necessity now to allow them to have certain freedoms.
I just want something they can use to call / text (was debating WhatsApp too) and I'm happy to have PAYG. I don't want them on a contract particularly.
Any suggestions / recommendations?