I was going to say don't have an aisle seat in the stalls as all the animals race down the aisles at the start and can be quite frightening!
We took DD for her 4th birthday, she had been to many plays/performances of childrens (and adults) theatre so was used to the whole set up, including sitting still for the required time. I think that was just old enough.
Mary Poppins is for over 6's I would say, took DD for her 7th and she was freaked by the Playroom scene - it was very scary and unpleasant.
How about The Snowman at The Peacock theatre, DD did this aged 3 and loved it.