I see what you are saying about not knowing the parents (for one of the sleepovers) well, @Newgalintown, and I do sympathise. There comes a point where children start to move towards more independent friendships - when they’re little, all their friendships grow out of things we as parents do - taking them to baby groups/toddler groups, visiting our friends with children who are similar ages - that sort of thing. Then when they start school, they start making their own friendships, where we don’t know the child or the family - and that is a bit disconcerting, as the parent.
I remember the first time Ds1 went to play at the house of a friend whose mum I only knew from chats in the playground - I had talked to her, and seen her interact with her child and with the other mums, and I felt confident enough to agree to the play date - but I still worried.
To my mind, the sleep over where you don’t know the parents very well is the next step on from that, and I can understand why you are thinking your dad is too young. I do think it is a natural next step, and you can’t put it off forever - once she goes to senior school, she’ll make friends and you may hardly even see the parents - but equally, it is reasonable of you to want to put it off until you are happier with it.
You said your dad doesn’t have a phone - is there possibly one she could borrow, just for the sleepover? Or could you arrange to ring her at bedtime, so she knows she will be able to ask you to get her, if she wants to come home, and you get a bit of reassurance too?