We like this resort and have staye there many times. Last year DS was 5 and enjoyed it. Liked the slide very much and thought the playground was lots of fun.
The bldgs you mentioned were all great. You will have to stay in the AB section as the mansions don't have trundles. We stayed in bldg. 15 and really liked the location. As of last year, they had not assigned any rooms as "preferred" and were not charging more for those. This may be new, though.
You can use the facilities at the POFQ and your younger one will really like the pool area there are it is much more festive than POR ... however your older ones will prefer slide and playground at POR.
You can also fish at POR. You pay for the pole and worms but it isn't much. Our DS had a lot of fun. Can't remember if we caught anything. You throw it back so it isn't bad.
Be sure to take the boat to Downtown Disney. It is a nice trip and the kids tend to enjoy it. At night, depending on the time of year, though, it can get cool on the boat.
Also, the lounge at Riverside has a great entertainer and it is truly family fun. Great for everyone, all ages. You should try to catch this if you have time.
Keep in mind, there is a path that runs between Riverside and FQ. There are lots of ducks, etc. along the way. If your kids don't mind walking, it is a nice little outing. And, you can also rent scurry bikes that you can aride along the path...adults can peddle and kids can ride. Might work.
We typically caught the bus at the bus stop just outside the gift shop in the main building/lobby area. It stops at POFQ first, and then there. Sometimes by the time it got around to the other stops it was full and riders had to stand. Might be something good to know if it is busy when you are there.
Food court, at POR is very nice. Lots of selection. A little bigger than FQ but again, you can walk down to FQ and use their foodcourt. The refillable mugs are good at both POR and POFQ which is a nice perk.