Movie passes are a great idea- it's a luxury these days b/c it's a fortune! They don't need to know that they were free. Just make sure that the theater is somewhat near their home.
Otherwise, a catered breakfast or lunch is ALWAYS appreciated. I had a parent do this for me one year (I run a "lunch bunch" for a group of boys) and it was a huge hit! She sent all of the service providers an invite to have breakfast in my office (she asked first) and it was wonderful. Bagels, juice, cream cheese, fruit and a beautiful hand-written thank you for each of us from her child.
If you are looking for "protocol" regarding childcare gifst, most people suggest one week's salary for the primary caregiver and smaller denominations for the aides. That can get very pricey, so do what you can. This year, we are doing a gift card to a day spa for the primary teacher and Dunkin Donuts gift cards for the aides. It's the thought, not the price tag!