Bit late on this thread but I wanted to add some valuable info. Staying at the Swan in August 2019, 2 adults 2 kids. The Marriott site does want to charge me the extra child fee of $25 per child per night. (Keep in mind the rate is only $125 so this represents a 20% premium per child!) Instead of paying this, I booked just the 2 adults with queen beds.
The next gotcha was when I linked it to the
MDE it only syncs with 2 guests. This will become a problem once I get into the 60 day Fast Pass+ window. I called Marriott customer service and they said since the merger they have had this problem with some of their hotels and children on a reservation. She claimed Marriott will never charge for children. To rectify this situation she needed to call the Swan because she couldn't access the Disney MDE system. After 15 minutes she came back and said give it 2 days and the additional guests will be in the MDE. Big surprise here, it didn't work and now my hotel reservation was completely de-linked from the MDE.
Called Marriott again today, same story about not supposed to be charging for children at this hotel. This time instead of calling the Swan I asked if she can just add the 2 children to the Marriott reservation, this way when I re-link my hotel stay in the MDE it will show 4 people. After another 15-20 minutes and speaking with her supervisor they couldn't waive the fee in the system. Told me a trouble ticket has been attached to my account and she called the Swan to confirm they will remove the $250 extra person charges upon checkout.
So tomorrow once the Marriott reservation is visible to the MDE I will try this again. This time with the 4 guests. However, I do have a feeling I will be dealing with the front desk at checkout to fight getting these charges removed.