How exciting! Ask if their agency has guidelines. If not, I'd include how long you've known them and in what capacity (friend, coworker, etc). If you have kids, describe how they interact with them. Include characteristics that would make them good parents.
Above all, be timely in getting it in. There's nothing worse than having your adoption paperwork complete but be missing a reference.
I will echo what Dr. L said, just let them know how long you have known them and how you have seen them interacting with children.
Please, please make sure that you send it in promptly. My home study was held up for a month because one of mygood friends just couldn't find time to fill out the reference letter.
Thank you for being a part of the adoption process!
Mother of two wonderful boys! Blessed through adoption.