Voltage relates to speed and power. The do make 24 volt systems, but they are mainly useful for flat street riding, 2 wheel bikes, and a light weight rider over short ranges. who wants
a little assistance sometimes. Its just not something I'd recommend for a trike, or off road, even with a light rider who needs
assistance most or all of the time.
Higher voltage often means more speed, but it also means more power and capabilities. 48 volts is what I would recommend for an adult who needed
the assistance. 36 seems about right for a child. If the speed is too high, the CA can limit the speed, but that may not be needed.
SLA, Sealed lead acid is heavy, and inefficent. Due to peukert effect, you'll need almost twice the capacity of battery for SLA as you would for Lithium. a 36V 10Ah Lithium (A123 round cell packs) will weigh 8-10 pounds depending on the source. you would need a 36V 20Ah SLA to get the same range, which would weigh 40-50 pounds. thats a lot of dead weight for a 55 lbs child to handle. the motor will move it forward, but she still has to steer and brake that weight.
And SLA has a very short life, even when taken care of. Good Lithium can be twice the cost, but should survive 4 times as long Minimum.
As for handlebars, those are cheap and easy to replace. Places like http://www.choppersus.com/store/product/469/Handlebar-Cruiser-Round-CP/
sell cruiser bars that are lower and come back farther, and may be easier for her to reach.
Buy the ticket, take the ride.