I think this is a good point. It's hard for me to fault Hilton here. Needing to add the names of children is not something necessary for 99.9% of hotels. I suppose it's possible there might be a few other hotels that need that sort of functionality. But, I also wouldn't be shocked if it was only the three Disney World Hilton-branded properties that need kid's names. That's 3 hotels out of almost 6,000.
Needing to ensure that guests can manually add names to reservations (either during the booking or after they are made) would be super low on the priority list. In fact, it might not even appear on the programmers' list of requirements.
aww3583: The three Hilton-branded hotels where this matters might be closed to guests, but they still will have some managers working. (They are still accepting reservations, so there has to be someone monitoring their inventory.)
Try visiting the Hilton Contact Us page. Select "Hotel Questions" in the drop down menu. In the comment box, enter a short statement. Something like: "Please add the following names to my reservation so I can access the Disney World benefits like FastPass+." Obviously, include the names, too.
https://help.hilton.com/s/contactsupport
It might take a few days, but I am hopeful that doing that will solve your problem.