How long is a bit of string, get a saddler or saddle fitter out to have a look and give you some professional advice. It's no good asking that sort of question on here. Youngsters come in all shapes and sizes, you should seek professional advice and do it properly.
Oz (Saddler)
We have 2 rising 4 yo that need saddles. One is already backed and the saddle I had especially made for Ruby appears to fit him. We are however waiting for a visit from the saddle maker to make sure that this is the case. Plus he is checking the fit for Breitlingh's and Norman's saddles. This is something I try to do at least every 6 months. Accenta is a different kettle of fish. She is going to be a big broad girl so we are going to get a cheap 2nd hand saddle from the saddle maker that we can keep trading back to him for other saddles until she stops growing and changing shape and then we can spend a bit more and get a really decent saddle for her.
I suggest that you find a decent saddle maker that will let you do the same otherwise you'll end up with hakf a dozen saddles you can't use.
You could get a really good treeless saddle - Heather Moffett or Solution for instance. I don't like treeless normally, but they adapt to lots of different shapes and as long as you don't ride in them for too long I think they are great. The problem for me is the stirrup bars - I can't see any way that they don't result in pressure points if you jump or ride for a long time, but for short periods when you are sitting in the saddle a treeless might be ideal.