We are a party of 5 also (me, DH and 3 DD's). We originally booked a cat 4 for our upcoming 4 day cruise, then had our TA change it to 2 connecting cat 10. Although we had to sacrifice the verandah, we were able to save about $550 and now we will have more space, more privacy and the part I'm most excited about...2 bathrooms!!
I have also heard of people getting an outside room either with a porthole or a verandah and an inside room right across the hall. Our kids are too young to be across the hall, but if you are traveling with your MIL, maybe she could take one or both of the kids with her and you and your husband will have your own room and everyone will still be able to take advantage of the verandah.