Tips to show appreciation

My in-laws take my 19 month old toddler twice a week for about 2-3 hours, even though I provide diapers, cream, extra changing clothes in the bag, I see them untouched. I have thought to give a bag of diapers as compensation. Is this ok?

Every season we get new clothes for LO and sometimes they bring cooked food over!

What else could you do to compensate for all the nice things they do for us?