K.P.
Let's see... 6 hours of babysitting for 2 kids. I would do a $50 gift certificate to local restaurant with a nice hand-written thank-you note. I think more than that may be a little extravagant, but less is a little cheap. Considering the fact that they will be at her home, so she can still get things done there, it's more convenient than being at your home for the day.