M.S.
HI B.-
I would recommend charging by the day rather than hour. $20 half day. (I charge $25 / $50 full day). I only do hourly if it is not a regular child and very infrequent visits. This is your time, your home, your food, your gas, exposing your family to germs he brings in from his school. If she can't give you $5, let her go back to her previous sitter. Most people will not pick-up/drop off at pre-school for $4 hr. One week alone is probably a 1/2 tank of gas! Just think of all the times you will be going out into the rain/cold. You are worth more than $5.
I STRONGLY suggest you get into a contract if you are going to be watching her son, regardless of friendship. The good child care out there would ask this of her. Especially since you are friends-- just imagine, Sorry, I didn't have time to get to the bank, forgot checkbook, etc... It happens. Make sure you have a late fee in there. Also, about sick days, etc... I have a sample contract if you want it.
Just by reading your post it almost sounds like you are doing this because you are kind and wanting to be a good friend and help, and maybe she's in some sort of bind. It sounds like she may be the kind of person to take advantage. If I were you, I would highly re-consider turning your life upside down with such a big committment. Do you really want to drag your 2 yr. old outside when it's zero, or snowy, if you don't have to??? Especially if you are not needing to do it for income.
Good luck. I've been taken advantage of (by previous families) and neighbors, It just isn't worth it. Your time is valuable.
M..