DS is 9 and got a Razor scooter a couple of years ago. You mentioned that your DD is not advanced with motor skills as a concern for being on a two-wheeled scooter. My DS has motor skill delays, low tone, and dyspraxia (motor planning disorder) as part of his autism, so his skills are like that of a much younger child. He still has training wheels on his bike.
He loves his Razor scooter. Riding it has really improved his core strength, stamina and sense of balance. The APE (Adaptive Physical Education) teacher has been amazed by how much his scooter has helped in these areas.
I don't know how much motor skill issues are a concern with your DD, but I thought I would share our experience.
Gena
DS, age 11 and always amazing
“Autistics are the ultimate square pegs, and the problem with pounding a square peg into a round hole is not that the hammering is hard work. It's that you're destroying the peg." - Paul Collins, Not Even Wrong