to pay the neighbors or not to pay the neighbor?

So my husband and I have tickets to see a show on a weeknight, we live an hour away from the nearest family member so that's out of the question. We have really nice next door neighbors whom I have asked in the past to watch our son for small amounts of time. this week, they are our only options. I called and asked if they could come and watch him and they said yes. So problem solved but now I'm having a dilemma on how to repay their kindness. They are an older, retired couple. I offered to leave them dinner, but she said she would have it prior to coming over. What is an appropriate thank you/payment for watching our son from about 530 in the evening (his bedtime is 7/730) to probably 10? Thanks for your vote/advice!