As others have said, car seats aren't allowed on the busses. If you're taking the van to SSR, though, you could either bring your own or Disney will provide you with whatever you need for the short ride. One less thing to pack!
Just make sure the seatbelts fit your boys ok. Their knees need to be to the edge of the seats and the lap portion needs to not be over their abdomen. The shoulder part needs to be across their clavical. If they don't fit these - then please go to Wal-mart and buy 2 of the cheapest boosters they have. (you can get backless boosters for under $20)