Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [first names of couple],
I know that when you have a small child in the house, it is very hard to get any time alone together. It is especially hard if you don't have any family living nearby, or if you have limited family living in the area.
I've been there, and wanted to offer my assistance to you if you needed it. It can be very hard in this day and age to find a babysitter you can trust, so that you can enjoy a few hours out with each other.
If you ever need someone to babysit for an evening so you can take a little time together, please let me know. I would be more than willing to help.