If your're going to the Mayan Beach side, you won't get buildings 1800 or 1900. They are on the Riviera side. Both sides are identical, just located one beside the other with Riviera being a closer to restaurants, theater, etc...
The dive shop will have vests for the children and snorkling equipment, but I think the time is restricted to 45 minutes of use per day. We brought our own equipment that we bought at Costco.
You will love the resort and our 9 year old loved it too! He also enjoyed the kids club very much. The staff there was very nice and trilingual. We are going back to the Grand Sirenis for the third time next December.