A hemi mom in a group I'm with, asked how our kids did on swings because her 9 year old couldn't quite do it. Since Blondie is also 9, this is for her. Blondie has been doing this for awhile, but I think it basically comes down to practice, practice, practice. She is also a left side weakness, so she says she uses her right leg to really get going, but she tries to move them both sort of at the same time.
I hope this helps your daughter.