Well how long was the cast on? Because she wouldn't have used her leg muscles etc. in all that time right? So they are weakened. Totally normal. Will come back.
Was she given any kind of exercises to do to help strengthen her leg muscles - sometimes they do to help regain strength and mobility. Maybe they don't with little ones.
Usually they recommend abstaining from strenuous activities for as long as the cast was on. Roughly. So that gives you an idea how long it takes for it to fully heal/strengthen.
I'm sure it's all completely normal. They check to make it's healed correctly, and will again when you visit.
If you are concerned at all, book an appointment. Sometimes people use canes for a while after a cast removed, or they get a splint for breaks that have been in a cast. I'm guessing they don't feel it's necessary, so personally - I wouldn't be concerned.
If you want physical therapy .. then ask the doctor if they recommend it. They would advise the PT on what areas need to be strengthened.
Good luck :)