I would like suggest like others to not bring one and make it fair, but it sounds like your kids are exhausted by the end of the day. So personally I would bring a single for just your kids who may need it. Should you bring a stroller for your friends children? DEFINITELY NOT!!!
Your friends by now should realize that they need to take responsibility for their own children knowing how tired they are at the end of a
Disneyland day. You have your own children to look after and care for, and items such as day packs, strollers, jackets, snacks, etc to lug around for them. You should not be responsible each trip to pack for another family as well. Especially if they feel their kids are too old for a stroller.
So if I were in your position, trying not to step on anyone's toes to hurt feelings or be judgmental, I would say to my friends
"Today I am packing lightly to make it easier on myself", but you can always ask them
"Do you want my umbrella stroller?". If they don't want it and they see you have a single stroller and your own children are using it, and there is no room for their tired kids, you can always suggest a stroller rental.