5 day trip to Cancun with 3yr old

I am planning a 5-day trip during the first week of September and need help in finalizing the trip.

1. Is it a good time to travel to cancun. Will it rain a lot during this period or it doesn't rain much?

2. Looking to book an all-inclusive hotel from the below list and need help in which would be better with a 3yr old, has a better beach with less seaweed and has a shuttle service

The Royal Sands Resort & Spa All Inclusive

Fiesta Americana Condesa Cancun All Inclusive

Seadust Cancun Family Resort - All Inclusive

Grand Fiesta Americana Coral Beach Cancun

Grand Park Royal Cancun Caribe

3. For local tours and activities, is it better to book through the hotel or from a local agency?