Definitely plenty of drinks and easy snacks, but a meal may not be necessary depending on what time they'll be there. If you want to provide lunch, I would stick with take-out/delivery of some kind, because yeah, it's silly for you to go to more
work if they're coming to reduce
Have some task lists ready--think about not just priorities for you, but things that can be done by different "types" of people, which things can be done at the same time, etc.. That will help things to be more organized and more will get done.
It might feel awkward to just sit there while they work, but on the other hand you don't want to get in their way or make them spend more time worrying about you doing stuff, so maybe have some quiet, sit-down tasks you can be doing (assuming you are able to work at all). If you don't need to be resting undisturbed, be available to answer their questions about things. Will they be bringing their own cleaning supplies or will you need to provide anything special? You might want to find that out ahead of time so you can stock up.
Also, be prepared for stuff to not get done exactly the way you want it to.
It sounds like that won't bother you too much, but I just thought I would mention it.
A thank-you note to each afterward is necessary, I think. I don't see anything wrong with a small gift as well if it won't put you out too much to buy/make it (like if you were going to bake or do a craft anyway, make a little something for each lady). It really depends on the personalities/relationships
of all involved--if you're the type to feel the burden of reciprocation keenly, the "extra" of the gift might alleviate that and allow you to feel that you are no longer obligated to them.
Also, and I apologize if I'm overstepping or suggesting something obvious here, but... if you are looking for ways to get your husband and older children (all school-age, right?) to help with chores more and with greater efficiency, I'm sure a lot of people here would have good ideas. If your finances allow it you could also look into hiring an individual to help out, perhaps a college student available during the day for a few hours.