Can't believe I've only just found this thread!
My eldest son has Down's syndrome, and unfortunately he can't ride a bike. Or doesn't want to!
He's 14 now.
We tried many times with bikes, trikes, stabilizers etc., the only thing which worked for him was a 4-wheeled thing, a bit like a go-kart. Very heavy though. He soon lost interest.
I was convinced that a recumbent such as a KMX would be good but he showed no interest at all.
I think it may just be down to being extra patient and allowing a lot more time. An ex customer of ours son has downs but once he got to grips with a bike of his own and balance etc, just rode and rode and rode......with a riding partner to add safety when needed.
That's impressive and very encouraging, as I don't know any kids with Down's who can ride a conventional 2-wheeler. Plenty ride trikes etc.. though. Even with Down's, the spectrum of disability is very wide, and there can be physical issues such as poor muscle tone, trouble with fine motor skills, etc..etc...
As you will know already, the attribute any parent of a child with Down's needs most is patience!! I think parents of disabled kids should be given medals, let alone benefits!
Please keep us posted on your little girl's progess!