We've stayed in a LK suite twice and a Nemo suite once and and don't think the bed can be moved. It's on a pedestal. I will say I've never actually tried to move it so maybe someone who is there now or is going soon could try to see if it moves. I think you're definitely going to want to buy 2 individual mattresses as opposed to a double. And if you plan to put both of them in the bedroom, I'd probably try to buy one of the toddler size ones for the 3 yo if s/he will fit on it. I think you would be able to fit 2 standard single mattresses in the bedroom but it's going to be tight. I did measure the distance between the bed and the wall and it is 38" (a standard twin air mattress is 39" wide) so you *might* be able to squish one between the bed and the wall but obviously the person sleeping on that side of the bed would have to crawl to the foot of the bed to get out. *MAYBE* a double would fit between the bed and the bathroom but I wouldn't chance it unless someone gets exact measurements because you've got a night stand and a slanted wall to work around. *IF* a double would fit there, you would not be able to get in or out of the room and you wouldn't be able to get to the bathroom without stepping on the mattress. I did drag one twin-size mattress in the bedroom to see how it would fit between the bed and the bathroom because it always looks like there is a ton of space there. Unfortunately, it blocks both the bedroom and bathroom door to some degree. Even if you put it tight against the bed, it will partially block the door coming in to the bedroom (you wouldn't be able to open it very far. I think it was too small for a person to fit through) and would partially block the path from the foot of the bed to the bathroom. The bathroom door was a little easier to deal with since it opens in to the bathroom rather than outward to the bedroom space, but you're going to have the challenge of getting around the mattress to get there. The person sleeping on that side of the bed would have to crawl to the end to get out of bed *and* they''d have to step over at least part of the air mattress to get to the bathroom (as would the person on the other side of the bed). I *think* an air mattress will fit at the foot of the bed, but I didn't write down those measurements so I don't know for sure. But the mattress would definitely take up the entire space so no one would be able to walk through there or open probably the bottom 2 drawers on each side of the dresser. The bedroom is a great size and it looks like it has a ton of space...until you try and put an air mattress in it.
Another thought -- would they be able to share a single mattress sleeping at opposite ends? My kids used to do that when we'd visit relatives who only had one single bed for them to share -- each kid would put their head at opposite ends of the bed. Their feet would touch if they stretched out, but they usually both curled up in a ball to sleep so there was plenty of space. And one other thought (though this is a little more challenging depending on the logistics of your family), my daughter loves to sleep on the sofa cushions from the sofa bed in the living room. If you take the seat cushions off and turn them upside down, it's almost like a little "trough". I call mousekeeping to get extra sheets and blankets and make a bed out of it. I've heard/read of other people doing the same thing. It's smaller than a twin mattress and would probably fit easily between the bed and the wall or at the foot of the bed with a little room to spare. Either one of your children would probably fit on that (my daughter is now almost 5' tall and she still used that for a bed this summer). It works for us because our son who sleeps on the sofa bed always has the bed open because it's easier for 2 or 3 kids to lay on the bed rather than sit on the couch to watch TV. It might not work as well if the other adults in your party either want to use the couch to watch TV, or if they are using the sofa bed, they'd have to open it up whenever your kids are ready for bed.
We use 2 twin mattresses every time we stay in the suites (4 kids -- tween through early 20's -- none of them like to share a bed) and there is room for 2 twin size mattresses out in the main living space, but I would not want to have 2 of them in the bedroom if I could avoid it. Personally, if I needed to have 2 kids sleeping on air mattresses in the master bedroom, I would be searching online or in a local store for youth or toddler size beds and if you're flying, have them (and an air pump) shipped right to the resort. I'd also take packing tape, some paper/labels, and a marker to reuse the shipping box(es) to ship everything home at the end of the trip. Or depending on the cost, just leave them behind or try to pass them off to another DISer who might need them.