You're going to find it pretty tight IMHO. You will find it a lot more comfortable to have two rooms, you do also have a number of other options. I would at least consider either the cabins at Fort Wilderness if you HAVE to stay on site. If not I would look at either an off site condo style hotel/timeshare unit or something like the Holiday Inn kids suites that would be perfect for your size of group.
While it may seem like "it's only a copuple of nights" and it's not too much of a problem I would take the view that it could be used as a relatively cheap way to do some research for a future (longer) trip to check out one of your possible options for the future. OK it may cost you an extra $200 to stay at the cabins, but if decide this is (or is not) an accommodation style that you would use in the future, it could be money very well spent. If it is a good choice for you, you will know you can happily book a longer stay in the future, if it's not for you, then you won't have wasted $1000+ by booking a week there at a later date, it's a cheap lesson. Again staying off site, isn't for everyone, but for families of 5, Disney's offerings are a bit limited. It's a question of priorities and trying a couple of days off site, in rooms that are better suited to your family will give you a much better idea if it's a worthwhile payoff. You lose some accessability and convenience, but you get rooms that will comfortably sleep all of your group. Your other option, if you were going to be staying midweek, would be to rent points from a DVC member and take a two bedroom DVC unit. Even this (cheaper than renting from Disney direct) route is going to cost you about to $300 a night, but it will tell you if this is something you'd want to consider in the future.