I too have found it much easier to do a childcare swap with just one friend whose daughter is the same age as mine. We've been doing this for about 6 months now. The girls are 21 months and they really enjoy it too. (The girls have been asking for each other for a week or so now because we took the last two weeks off while both families were traveling for the holidays.)
I think the main thing is to work out ahead of time how formal you'll be in terms of hours and days, house rules (ie-no tv, what type of snacks and food), what to do in case of emergencies, and then just try it.
Happy with my DH, 2 kids, dog, fish, and frogs