Easiest way of getting about Paris with any kind of pushchair or wheelchair is actually by bus (and the buses are good and have flashing displays telling you the name of each stop). Unfortunately you will have the usual problem that not everyone will recognise that a mac major is for a child with a disability and won't necessarily give up disabled spaces on the bus for you. The RER and the Tram are much better for getting about that the metro (not sure that the mac major fits through the turnstiles). Have a nice time!! BTW if you have any sort of proof of disability you can use it to get into most museums free.
On a metro you need to take the main entrance ie one where there is a ticket booth- there you'll have a door that the ticket booth person will open for you, so no need for folding the pushchair to go through the turnstile.
I find that people let you use the pushchair/disabled space on a bus. Certainly there isn't expectation that you have to fold it as it seems to be in the UK.