You could start to limit their morning nap to just an hour. I did this with my ds when we started having problems with his pm nap. Or, you could start transitioning them to one nap. When they get tired at 9 try giving them a snack to give them more energy, and then you could put them down for the one nap at 11 or so, and gradually push the time back bit by bit over the course of a couple weeks.
Hope this helps a bit!