It would be helpful if they spell it out, but they don't.

They also use the term "twin beds" for both double and single beds in some places. I usually check the hotel photos, check tripadvisor comments or call the plat line for more info. The plat line has called the hotels directly to verify whether 4 would be allowed in a room with 2 doubles.

Marriott doesn't have the kids stay free in existing bedding or whatever rule, and it depends a lot on where you want to go, which you don't mention. As RogerD posted you are usually ok within the US, Canada and Caribbean. I say usually as there are some exceptions, for example the JW Marriott Marquis Miami has 2 doubles and states max occupancy at 3 people.

I have discovered that in the UK, if the hotel has 2 doubles they will often allow 2 kids when I ask. The Marriott booking system will say 2 guests max however, so you won't be allowed to put 4 in the reservation page. In France hotels with 2 doubles would not let me book 2 kids and 2 adults. Thailand would let me, but only if both kids were under 14. China depended upon the hotel. Etc. etc.

And some people have booked 2 guests and shown up with 4 only to have a very nasty surprise at check in. So, do your homework first. However one of the main reasons that Marriott remains our primary choice is because many international hotels do have two doubles in a room.