N.B.
You are very generous and your caregivers will appreciate your thoughtfulness and generosity. You will likely get a wide variety of answers and this will depend on the areas and economy of where people are located.
I live in MN and do home childcare. Some of my families over the years have done nothing, some do a small $25 gift card to Target, bookstore or a restaurant they know I like (Olive Garden or a local place I might have mentioned, etc). Some have bought me things for the daycare (like books or a toy I mentioned I longed for for our group..or a gift card for the local educational supply store. One used to have a catered breakfast delivered to us every year on the last day of daycare before whatever break I closed for.
Some were greatly generous and gave me cash equal to a weeks pay along with a small gift (like a coffee mug, fancy chocolate bar or a home made dishtowel set or ornament). Local salon/spa certificates with those higher amounts have been received as well.
My point is, everyone can give differently...you should do what feels right and that you can afford to show how happy you are for (hopefully) the wonderful care your child is receiving. Cash or a very usable gift card along with some baked goods or a pretty ornament...or if you know something specific about them (if one loves coffee....or mentioned something specific?) .In center care perhaps the interactions are a bit different than I am accustomed to with my own home childcare? But do what feels right!