I would make a specific plan to watch their kids (free) so that they can get errands run on a weekend without the kids in tow! Really, this would probably far more appreciated than anything you could purchase.
We have provided a lot of support to my niece and nephew (same age as us, wacky family structure) over the last two years due to some significant events in the family. There is nothing that we "need", but they wanted to do something as a "thank you" and offered to babysit so we could go out without hiring someone. We thanked them and didn't act on it. A month later, my niece called and instead of "reminding me" said..."We'll be over on Saturday at 4:00 and will plan on being at your house until 9:00. Find something to do!" and hung up.
It was wonderful "found time" for the two of us and we ran errands, had dinner and caught-up with friends for a drink. Give the gift of time & it will be much appreciated!
You could also invite them over for a "holidy party" as a thank you. Order pizzas and hang out, if they are close friends they will love a night "off" from cooking and cleaning up dishes.