Hi Lisa!
Your twins will love the Contemporary. We stayed there recently with my children (6 and 2) and my daughter (the 6 year old) still talks about the Contemporary as if we there just yesterday!
Length of stay is tough. As a panel member, I would probably say a month, but that's just the "Mickey" in me talking-smile!
...But in all seriousness, anywhere between 5-7 days is a good fill of Disney. For example, we stayed last year 4 days and felt the vacation wasn't long enough. We stayed 6 days 2 years ago and felt pretty satisfied when we left. If you plan to visit all 4 Parks, you need one "off" day either to hang out at the resort, or check back one of your favorite Parks (Magic Kingdom, for example, always takes us the equivalent of 2 days to get through), so hence the 5 day rule.
As for booking, you can book dining/special events as early as 180 days before your visit, so you should be good.
As of this writing, 180 days would put you at around September 14th, so if you book anytime before that day, I'd start making reservations right away. The earlier you make them, the better opportunity you'll get the times and dates you need.
Best wishes to you and yours for a magical time!