Look at the Bangkok Marriott-family hotel thread on the Marriott forum. The wiki at the top of the thread lists the hotels and provides basic information about each, such as location, CL access, typical UGs and general consensus, both pros and cons.

The general consensus is the JW is not as nice as some of the other Marriott properties in Bangkok. Different people have different preferences, but most rank the Ren, Marquis and Soi 57 as better properties than the JW. If location is the most important, the Ren might be right. The Marquis is adjacent to Queen's Park which might be helpful with a kid. Personally I like the Soi 57 best, but have had great stays at all three. Each has a great CL, although kids are usually not allowed after 17:00, and the Marquis and Soi 57 usually UG to a two room suite with an extra half bath at Soi 57. Technically you cannot have a third person in the room - no rollaways allowed - but people have indicated the hotels have permitted this in the past. The Soi 57 has some small, good, inexpensive take-out places steps outside the hotel.