G'day SM,
I'm with you mate, i'd be going for the bigger 90cc version........ unless u and the Mrs are both real light weights and small of stature i reckon the physical size of the 50cc unit would be real uncomfortable after even a few mins, then there's the whole lack of power problem, and the bike will have a load rating, so i imagine the little suspension set up would be getting a bit of a hard time with an adult on board..... also, with an adult sitting well towards the back of the little 50cc, i reckon it'd be very inclined to want to wheelstand, even on the slightest incline etc......
I reckon for an all round more enjoyable experience the 90cc would be a better choice. See if you and the Mrs can take a couple for a sit test or ride....... i'm sure any good bike dealer will have dealt with this before and be able to give you plenty of advice..... And like you say, the 90 will be usable for years to come.
I believe too on most these little bikes you can get adjustable throttle locks, so you can set so the bike can't go much above idle, just enough to get it moving, and yep the remote kill switch is a good idea too.
A flexi stick with flag (pink flag of course) is a good idead off the back of the bike too, helps other riders / drivers see the little kids in between the hills and bumps around a track (obviously i'm talking about once the little one is skilled up to ride on her own)..... also when the bikes are parked, it may stop someone in a car etc reversing into the parked bike.
As you have probably seen with your boys, the kids (boys and girls) will learn quite quickly how to get around on the quads, mini bikes etc........
oh, and don't forget the pink streamers in the hand grips, little girls have got to have some bling!
Good luck!
Barrabart.