Hi Victoria, this subject has been visited MANY times and usually ends up causing (quite vicious) arguments. People who "bend" the rules on one side (certain that their transgression doesn't hurt anyone else) and those that feel "the rules are the rules" and should never be broken. Personally I tend to fall into the second catagory, those rules are there for a reason, we all knew the rules when buying into the "club" and therefore we should stick to them.
I do understand the temptation for families with 3 kids, a little pushed for points who want to give their family the best vacation possible. It's very frustrating when you have to book a whole extra room when you have three young kids who are quite contented to make do with the sofabed and maybe an airbed and a sleeping bag. I also understand the frustration of those who abide by the rules, only to see others flaunt it in their faces.
IMHO the best policy for people who fall to the temptation of not getting the correct sized room for their party is
BE QUIET ABOUT IT, and don't wind people up. . There are quite a few people who do have more than 4 people in a unit designated as 4 maximum. It only becomes an issue when someone says " I know the rules, and I don't care about them or what anyone else thinks, I'm going to do what I want", that tends to rub a few people up the wrong way.
With your family group, as has been pointed out, by the time the "legality" of this situation gets to effect you, your DD is going to be 9 coming up 10. Very soon she is going to get to the age where she may not be happy sharing a bed with her younger brother(s), you ALL may be very grateful that she has a bathroom to take her time in to get ready to go out and EVERYONE appreciates the extra space of a larger unit. If the two boys shared a bed, having the living room for you and DH to enjoy a glass of wine, relax and chat before bedtime might also raise the quality of the experience for the two of you ( as opposed to having to lurk in your bedroom or creep about the living room if you need a drink or somethng from the kitchen)
As AZKathy points out with some careful points management, it's possible to make the most of the points you do have, even if that means cutting a day off your vacation, or spending $$ on a room to avoid using points at the weekends. IMHO a 5 star stay for 5 nights is better than a 3 star stay for 6.
Don't forget a
DVC purchase is for MANY years and while you may face a time ( 8 or 9 yrs) between youngest DS making 4 years of age and DD hitting an age where she doesn't want to come on vaca with the family you will have a MUCH longer time where your points need would not be as high as they were when taking all 3 kids with you. In your situation, if I bought, I would try to manage the points so that I was borrowing points from the next years for the time you need a two bedroom (increasing slightly each year the amount of points borrowed) until DD hits an age where she may not feel like coming, and you could then start "repaying" those borrowed points (building them up for the time you become grandparents and want to take kids and grandkids away).
Maybe we just need to rethink our situation. As it is, we cannot keep our boys in their own bed at home, they prefer ours! 
We have been working on that! In a strange place, our youngest usually sleeps with us, as he has a bad tendency to get up in the middle of the night and wander around- not very safe in unknown surroundings!! Maybe we will just have to wait until the kids get older and decide then.
