I did an open house yesterday with another agent (doing my first solo one on Sunday) and we had 2 vistors, both looking for homes for their childern. I don't know what to say in the thank you card I'm going to be writing them.

Any ideas?

Keep it simple, thank them for coming to the open house and let them know that if there is anything you can do for them you would be happy to help..
P.s you may want to let them know of some other homes they might want to take a look at. Be sure to tell that YOU would be happy to show them to them.

This is an example of the thank you postcards I send out the day after the O.H.
Thank you again for stopping by:
.....
Might this be the right home for you? If so, please call me and we’ll set up a private tour. If not, I’d like to help you find your perfect home. Remember I can show you any home that’s for sale, whether or not it’s listed by me or my office. And I know you’ll appreciate having an experienced Realtor® by your side. By coordinating efforts with other agents, I can show you more homes in one morning than you could ever see in days on your own. Plus, I can help you rule out homes that don’t meet your criteria or budget. Your perfect home IS out there. I’d appreciate the opportunity of helping you find it.
Thanks!
3 days later I send out a survey requesting feedback for my seller. I receive about 78% of the surveys back.
Hope that helps.