Well, here's my two cents:) I have a 3 1/2 year old who didn't move to a regular bed until he was 3. He never really climbed out of the crib, so I didn't see any need to move him right away. We didn't have another child waiting to use the crib, so there was no urgency. When we moved him to a regular twin bed, we had the worst time in trying to get him to stay in bed. He was fine once he fell asleep, but until then, he'd be out of his bed at least 10 times, trying to see what we were doing, etc. We finally got an extra tall gate to put on the room, and that worked until my stupid brother was babysitting one night, couldn't figure out how to open it, so climbed over it, and my son figured out how to climb over it, too. So, we started a sticker "program" where we'd give him three stickers on a little chart when he went to bed; if he got out of bed, we took away a sticker. If he got 10 stickers in a row (which was really like 20, because he can't count!), he'd get to pick out a reward from a jar (like a trip to the park w/Mommy, etc.). This worked really well, I have no idea why!, but it did...and then we just recently phased it out and he's been fine.
SO, I'd say go ahead and put your daughter in a regular bed now instead of going to the hassle of moving another crib in for such a short amt of time. I'd transition your daughter first, and then deal w/your son. I'd try doing nothing extra at first, but if she discovers the freedom of climbing out, I'd implement some kind of system of rewards so she's motivated to stay in bed. I do still have the gate on my son's room, too--I figure if nothing else, it will slow him down in trying to escape, if he ever decides to. I told him it keeps the monsters out (he's going through a "scared of monsters" phase right now), and that's been enough to keep him from wanting it off the doorway. Good luck!!!:)