I had the same issue with my daughters.
Tried my 9'mal but it didn't have enough volume and was too narrow.
Looked at a couple of (well used) 2nd handers but that didn't excite them.
Finally bought a very nice 9' Fly from Warwick at RPS.
Very happy with it - good size for them, and I can paddle it when they want my Nalu.
I think if you're going to be serious about getting them interested you need to be a bit serious about the board you get for them