I don't want to

the thread, but I have always included a friend for my girls when we go on vaca.
My sister's kids fight like cats and dogs....boy and girl, close in age, absolute delights when they are away from each other. But if they are in the same room it is miserable to be around them.
I never ever had that problem, and I finally realized it was because we always had extra friends, they always had someone to play with besides each other, and are truly the best of friends today at 23 and 25. I wish my sister and I were so close.
And I always pay for it, except for personal spending money. I just count them as one of my own until we get back home.
That is just my own personal philosophy, I'm sure there will be many different viewpoints, and none are better than the other...