Holy cow, you must have the most boring life in the world. No one plans on having more kids than they can handle....Even a couple with 1 kid needs a night out by themselves once in a while, it makes them better parents and shows their child a healthy example of a relationship. Not everyone has an in town parent or relative to dump the kids with. OP didn't even say how many kids she has - between the 3 couples - there are 5 kids! -she might have one!
Not that it would matter if all five were hers. I usually try to ignore absolutly ignorant posts like this - but this one just had me laughing too hard. OP is not looking for someone raise her children , just be an extra hand to hold in the parks, another set of eyes watching...be in the room when the kids are sleeping so she can enjoy time with husband and friends. You should relax and ask your husband to take you out to dinner tonight (oh - thats right, you can't, only you are able to be with your children)
Anyway - OP - it is good you are thinking this out ahead of time... what ever you decide, make sure you are up-front with the sitter and her parents about what your expectations are and what you think fair payment would be.
I have been on both sides of this (as the sitter when I was young - and now as a parent) - I usually expect the sitter to be on "light duty all day" which means she is there to help take kids to the bathroom hold a hand in the park (or beach) generally be a mothers helper, but if there is a ride she wants to do she is free to jump on it and meet up with us (I actually end up having fun doing rides with the sitter that my husband won't do.) I expect full duty for a few hours 1 or 2 nights of the vacation, (for me to go out with hubby). I have the sitter share a room with my daughter. Never put a sitter in a living room on a pull out couch. It is hard enough for them to be with a different family for a whole week, they do need some privacy. I tend to take care of them like they are one of the kids, making sure they have a clean towel, are getting enough to eat etc. (Some kids are shy and afraid to ask for things they need - so be mindful of this and ask them once in a while if they need anything) I also give them my prepaid calling card to call home every night and check in with Mom. When we go to the beach with 3 families and there are 8 kids, we each take a sitter and give them sitters night out, when we stay home and watch movies and the sitters go to the movies or shopping or something.. (I don't know if you would want to do this since she is by herself) Anyway, I would never expect someone on duty 24-7. We pay for all food, trip, flight, and give about $100 for the week as a thank you. I usually buy a thank-you gift or souvenier that I saw the sitter admiring during the week and surprise her with it when we are leaving. I had concerns about taking a sitter the first time, but if you have a girl that you and your kids are comfortable with, it makes the vacation great! - I don't think I could vacation without one again. As far as spitting up the cost - I would think that if 3 couples are splitting the cost of her trip it really is not that expensive for you....so maybe each couple should chip in a monetary "thank-you" based on how many kids you have... one other note...one year we shared a babysitter with dear friends of ours on a trip. I had clear guidelines set up with the sitter ahead of time but apparently not with the other Mom. She used the sitter as a slave, even asking her to walk back to the beach house from the beach to make sandwiches for her and her kids!!!!! (appropriate use of a babysitter would have been to say, "can you take the kids to the pool while I make lunch" (you are still the parent), she also did selfish things like get up from a dinner that I had just made for her and announce that she was going out shopping with her husband and the sitter could watch her kids. (since all the kids were not in bed yet, it would have been too much for us to leave our 4 with the sitter too, so we had to stay home) needless to say, we don't vacation with this couple anymore (their kids are brats anyway) Oh that is another thing! I try to take my kids vacation into account as well....I only vacation with families that have kids that my children enjoy...it is an awfully long week when your kids have to deal with your friends brats.
Sorry this was soooo long winded - I'm looking at all the work I need to do today and I guess i'm trying to put it off a little...