Sunday check-in is green. However, we were given a "green" unit in Fountains when we checked in end of March on a Saturday.
It can't hurt to check in early if you can. IMHO it depends how many owners will be there when you go (whether you can get in Cascades or Lakes - they seem to hold these units for owners). Make your requests via the phone and on the Diamond Lane checkin on the website. I guess I've said it here before but we also liked the unit we had in the Falls area near the pool (right across the street). It was a bit smaller than the Fountains unit we had but it was very convenient to the pools, bar, mini-golf, activity center, etc. I would concentrate on the type of unit that you need (near a pool, top floor, waterview, etc) rather than have your heart set on a particular section. You can always ask to switch if you really hate your unit. We did this in March and were very lucky to be moved.
We have spent quite a bit time with our young child at this resort. We like to pool hop depending on the activities offered. (Depending on where you are you can walk to the pools or drive). My DS's favorite pool is the Falls pool. It has a small slide and it's much smaller - so therefore, not usually as crowded. Plus, the restrooms are right by the pool, making it easy for him to go - when he really needs to go! However, it is not zero entry - but does have large steps in two parts of the pool for little ones.
The Fountains , Lakes and Cascades pools have zero depth areas. Cascades and Lakes have the fountains in a shallow area of the pool (great for little ones). The Super Pool is near the Spas area. I don't believe it has a zero depth area - but I do believe it has large steps in the pool. You're never far from a bar at the Vistana - except maybe the Springs pool (don't remember a bar there). We like to pack a small collapsable cooler and tote our own beverages to the pool area.
We really like this resort - especially when we have time to just hang out there and enjoy the activities/pools.