B.B.
The biggest thing is the relationship the kids have with the person. Have a day that they can come in and "watch" the kids while you are there. Watch for the way the kids react to the person and the way the person deals with them....
Make a list of house rules and ask the person if they are willing to follow them while in your house...
List EXACT responsibilities (any pets that you would want them to care for, run a load of laundry, load the dishwasher, etc)
And have a clause in your contract that YOU can terminate at any time with out notice but they should give you at least a week notice unless there is a medical emergency on their part.
If I were hiring someone to come into my home I would also include a no short shorts, skirts or low shirts (tank tops, tube tops, spaggetti strap shirts). Its a JOB, not a paid vacation with your kids. But then again I also have an impressionable 7 yr old girl I am trying to raise to be modest with her body...