As the other poster indicated, only two fit. It is a very tight fit. My DH and I did NOT fit together in the same rocket. He is very tall (6'5") and I would consider us medium sized but not pooh sized (I've lost weight since - yea!

). Anyway ... one child and one adult should fit but not both.
But now I will tell you -- I think it is the most frightening ride I have ever been on in my life.

I felt like I would fall out. I don't think I would let a 3 year old on it -- it is very HIGH up! Now this horrible fear is coming from someone who rides any roller coaster -- those ones that go straight up and straight down - I LOVE THEM ... but I will
NEVER EVER EVER ride Astro Orbiter again. I lost a year of my life on that ride ... plus another year WAITING to board it. Very slow boarding.
I know there are others that love it and DH rides it still ... but maybe someone else will tell you whether it is appropriate for children the age of yours. I think it is a little too frightening. I'd rather get the same "effect" riding the Magic Carpets (or Dumbo but less of a line at Magic Carpets).
Edited to say: I re-read your post and apparently they have ridden it at DL -- is it up as high as at WDW??? Maybe your children are more fearless than me ... ha ha ha ... have fun!!!!!!!!!!!