Do it!
The only 'adult sized' furniture that I have in my dc area is my desk and chair and the dining room table and benches. Everything else is kid sized.
I started my daycare in a split entry; after about a year we quickly finished the basement and moved all the daycare stuff downstairs. I had two bedrooms, a 3/4 bath, the laundry room and a family room that had a bar-type area that I used as a kitchen area. We still ate meals in the kitchen/dining upstairs, but snacks were eaten in the daycare area.
I know that it will be alot of work to move things around, but I don't think that you will regret it. Certainly don't let the lack of adult sized furniture bother stop you.
Good Luck!