The teen seats would be too small, unless maybe you are talking about a very petite woman.
Obviously money is an issue on all of these things, but you could build two sets of rails/brackets for the cage mounted seat mounts, and bolt in the teen seat for the kids and an adult sized for when you are wheeling with an adult passenger. You could use the same five point for both, I'd think.
Shouldn't take more than 15-20 minutes to swap out before a trip, and then as your kids get too big for the teen seats you are already set for the adult.
That is exactly how I would do it so my kids can grow with the rig rather than the rig having to grow with the kids.