I'm a physio, though I haven't worked in paediatrics for quite a few years.

Most paediatric physio will start by distracting your child with various toys while she asks you a bunch a questions, at the same time observing your daughter to see how she moves, what her posture and balance and so on are like. She'll then do a more formal assessment, but again, most paed physios disguise this as just playing, and often the child has no idea they're being 'tested', which means they behave much more normally. Some of it will be seeing if she can perform certain skills, depending on her age, some will be testing reflexes and flexibility and things. I used to have a song for one test, I'd say, 'Oh! do you know the song about the sun coming up? I love it, it goes like this... here, let me help you do the actions!' and then I could handle their limbs as I sang. It's highly unlikely that there will be anything scary or painful at all, and most kids and parents find it kind of fun to visit the physio.

The treatment is usually in the form of more play, as Sararose said, with the physio teaching you ideas of the kinds of play/activity/interaction to encourage (often jumping, bouncing, weightbearing-type activities for hypotonia) that will help her to develop normally.

If she has another primary carer (?her father? her grandmother) it's not a bad idea to have them along too, at least to come in for the last part of the session, so that they also get to hear the advice and see the demonstrations of whatever the 'treatment' is going to be. The physio generally has more time for you to ask questions, and also they often have a knack for explaining things in ways that parents understand more clearly compared to doctors, so make the most of the opportunity and have questions ready!

I wish you and your daughter well.