Thanks for all the feedback. I'm going to charge a set rate and that's that. As far as liability, we spoke with our insurance agent. He said if it's not a business, its covered under our umbrella policy. 

You and your husband need to agree on a price for full care and a price with expectations for part/working off board. Then sit with your friends and discuss what you all are looking for from each other. After agreement put it in writing and sign it.