We let DD (14) bring a friend when we went for the marathon last Jan. It was a short trip, but the friends parents had to agree to let her miss a couple days of school.
I have considered it at other times, but often my BIL travels with us, and increasing travellers to 5 brings on a new set of problems. At 4 people, we can always squeeze into one room (we carry an air mattress with us) And many times we go involve missed days from school, which I find is more difficult for older kids, and often not allowed by their parents.
When we did it last year, we paid for plane, room , food. Guest had to buy their own ticket and have spending money. (the plane ticket I actually purchased through SWA credits on our flights)
FWIW, we hang onto our F&W wrist strap gift cards. So we loaded up two of the cards with $50 each and gave them to the girls to get snacks or whatever they wanted. - Better that than a room key, because it maximizes the $ to be spent.
I made it clear to the other girls mother, that we may let them go on their own around the parks (as long as we were in the same park or resort area) Since our home resort is BWV, they could go to EP or DHS and long as they told us they were going.
It was great fun having a new set of eyes with us.
We have talked about bringing DD 12 a friend, but I am afraid my DD 8 will feel left out.
momx2, I only have one, but I think you'd be better off letting the 8yo have a friend then the 12yo. We have friends we visit in FL that have a 13 yo daughter. My DD just turned 15. Sometimes they get along, and sometimes my DD would rather be alone than play with "such a kid" Unless you're planning to entertain the 8 yo full time, I would not suggest bringing a friend for the 12yo JMHO
Too bad you can't bring a friend for both!