I was a daycare teacher prior to having kids and I watch my cousin's kids in my house now. Honestly, just anything that shows that you remembered and care would be nice. Last year I watched them on Christmas Eve, against my better judgement to even do that at all but I was trying to be nice so I did it. That kid puked all over my house and when I called the parents and said to come and get her, nothing. No card, no gift, no "merry christmas", no offer to help clean up the barf (they are not supposed to send their kids when they are sick, let alone sick and on a holiday). So here I am, already stressed about the normal holiday family demands and I am scrubbing barf off of everything and washing barf laundry instead of being with my family. Super. Oh and I gave gifts to their kids ($15-20/kid) and nothing- not even a card.
Just do something, anything. Even if it's just a card with a note saying how nice it is to know that your kids are in good hands and how much you appreciate that. It's largely a thankless job and it's nice to feel appreciated every now and then.