Can you imagine the kids climbing down from the uppers with a bed in the middle of the floor? My friend wanted us to cruise with his folks in one cabin and I decided I would have refused to go in that case. Maybe get adjoining cabins if you can afford it. Just remember too, what it will be like to share the bathroom. A schedule would have to be made for showering and dressing. I was lucky as I was an early riser, did everything while my friend slept. I saw a pic on someone's site that showed a rollaway sitting in between the floor beds, which the steward would put away in the morning. But that would only accomodate 1 child. You have a tough decision to make.
A lot of people put 4 adults in one room. Sure it's cramped but if you're not going to spend a lot of time in the room and can manage with one bathroom then it shouldn't be a big deal. Usually it's 2 uppers and 2 lowers and yes, the lowers can be pushed together to make one bed. The upper beds are very comfortable and the kids should be fine in them. The rooms are small but it's no worse than if you go camping together. Who spends a lot of time in their room anyway? The biggest problem would be the bathroom, if that's not a big issue then one room should be fine.