I know the child is getting too old but depending on size will she still fit in a jolly jumper as that would allow her to be in an upright position but exercising the feet as she will need to push off the floor. You can also do a lot of exercises while in the jumper.

Another thing to look at is the hips. I had a child that was born with dislocated hips and had to wear a brace at sleep time and she was nearly 18 months before she could start to go without it. I'm sure the doctor would have picked up on something like that soon after birth but it never caused the child any pain really just didn't have proper use and support of her hips because they slipped around in the sockets so easily.