Thanks you guys...
I really like the idea of movie tickets for K's mom, as well as bringing dinner over, or maybe even a restaurant gift card. Movies are a fun treat for everybody, right? So they give us a night out, and then we give them a fun time out on us! AwaitingBeloved, your comment about feeling less special somehow if money was part of the deal is exactly what I was trying to express. You said it much better; it helped me clarify my feelings of unease about the whole "payment" thing for K's mom. I think a gift certificate or two is a nicer way to say "thanks" in this case.
I think K is a little old for me to exchange babysitting, per se, with her mom, but having K come and hang out with me if her mom needs a night and her dad isn't available is something to consider, too!
Jenna, thanks for reminding me to discuss any payment for K with her mom first. I'd obviously planned to ask her if it was OK for K to be my "mother's helper", but totally spaced asking what her mom thought was appropriate for payment! That's important; I need to discuss all the aspects of this with her.
The only thing I'm really concerned with is making sure they know that I don't
expect them to watch him, because of our relationship, you know? If they feel comfortable and I'm not imposing on their plans, then I feel OK asking either or both of them. BUT I never want K or her mom to feel like I'm taking advantage or pressuring them because she's the big sister. Does that make sense? I just want to be really respectful.
That's part of why I want to pay K and at least offer something nice for her mom if and when they watch Bean. I want them to know I appreciate them, that I know their time is valuable, too.

Lulu, your post about how your 10 yr old bson would be a great babysitter, but that your situation right now wasn't in a place where that could really happen struck a chord. I do cherish the warm, "easy" relationship that I have with K and her family.

I always want to do my part to respect it! I hope things get better for you.