OK. I see what you are getting at. I would recommend on yourself going to 36v. It will carry you easily enough to keep up with your daughter. Mine is 11 and I won't let her ride 36v because it is so torquey! It can throw her off if she hits the throttle too hard. As far as range I got around 30min @ 12mph and on 36v I got about 15-20min @ 18mph. I could get almost 4mi range. I also used a car charger to be fully charged in 80min. My throttle was a twist on, let go off so every time I hit the throttle it would wind up slowly taking a long time to get up to speed. My 1st mod was using an 80t sprocket on the rear tire. I had to lengthen the chain, but the pickup was amazing. Top speed in half the distance. This would also make 36v more efficent as the motor gets up to operating speed faster.
Check it out,....
1st find out what kind of throttle you have... an adjustable throttle is needed
charger is no problem.http://tncscooters.com/product.php?sku=101140
all you have to do is connect your charging port harness into the controller's charging harness.
this thing will run 5 7ah batteries...60v. I'm running 2 of these controllers on 60v. The 4 and 5 wire models.(which is cheaper than the 5wire model.)
Now there is a catch, if you need to get the adjustable throttle you will need to put the pins and harness together, but it's not impossible.
Get 3 of these and put the 2 that you have up on the shelf. Keep them topped off with a 12v trickle charger 1 a month.
Then if your daughter wears out the 1st set you have a 2nd set.
Batterieshttp://cgi.ebay.com/12V-8Ah-SLA-Sealed- ... 2a0d7d17f5
These will fit in your deck just fine. The 3rd problem you will have will be mounting the 3rd battery. When I did mine I hung a cooler bag from the steering tube. I would now strap it to the rear fender with a ratchet strap. You will really enjoy the scooter, and your daughter will get a kick out of a simple sprocket swap....you could also swap your rear sprocket. That's what I did and the hill climbing ability was awesome.