Though you don't plan on meeting in the parks, one of your easiest options would be to meet them at CR on a day you plan to visit the MK. Go directly from MK to CR. If you are just looking to sit down together, the lobby has seating. Though you said you have been over there, it can still be relaxing to stroll the walkways all around past the Garden Wing. Maybe hop a shuttle boat?
Likewise, they could meet up with you on their Hollywood Studios day. If they are coming from their hotel or HS, then you could easily meet up somewhere along the Boardwalk. Lots of options all along that area! Ordering a pizza, ice cream, or something from the new deli would all be options if you want to snack (no ADR's). Maybe pair that with a walk. Likewise, it would be pretty easy to visit the mini-golf. There is space to hang out over by the Swan/Dolphin. You don't need reservations to get ice cream at the Fountain (Dolphin) or to sit in the lobby area. The little ones might enjoy seeing the big Swan fountain in the Swan lobby.
There is also seating by the water/campfire area at the Beach Club. During the day it is usually empty. Everyone is allowed to eat at Hurricane Hanna's, if you want another place to sit/snack near your hotel.
If you are transporting children in a car, of course you have to have enough car/booster seats. If you only have a 1-year-old and they have a 6-year-old, you'd need to have a booster seat for the 6-year-old.
From CR it isn't super quick to get to Disney Springs, but that is a good non-park place to go with little ones. Again, with plenty of places to sit and many snack options. Blaze is inexpensive and there are lots of places to sweets.
I have not visited, but don't the villas also have a game room area? With games you can borrow? A Movie Under the Stars might be an option at either hotel. I don't think the hotels have any restrictions on who can sit to watch the movies.