Thanks for the responses...
Well, I called... and they said they don't charge the extra. I was told just to book a room with two beds and put two adults in the occupants field... and everything would be fine when I arrived. The person that helped me said that they don't charge it, but it was part of the new Marriott system. Which seems odd, because it's not like every Marriott charges - clearly there's a way to toggle the extra person charge on and off for any given property. But in any case, it's nice to get to avoid sneaking my kids in.
Unfortunately, I bet there's a fair number of people just paying it. Hopefully this info helps someone in the future.