I am an only child and let me tell you how much it meant to me that my family invited other children to come along on our trips. I loved having someone my own age to talk to, to point things out to, to compare and just giggle with. It didn't take away from the time I spent with my family, but it was great for both of us. The things my mom or nana or grandpa didn't want to do, we could do and they could rest or have a snack and just sit in one spot while we did what we wanted to. Or they came on stuff too and we just took turns sitting with each other.
Now I have 5 kids,

and we don't have much room for others. I did just have my first grandbaby who will start traveling with us soon. I have made separate day trips for my 11 year old son to take a friend somewhere or my 8 year old daughter to take a friend ( the others are 22, 20 and 1 so they either don't come anymore or don't know they can ask to bring one, lol) and it's alot of fun for them. I don't feel like we feel left out at all.
I have never asked for money, but I haven't gone that long either. If someone invited one of my children on vacation I would NOT be insulted if they just asked for ticket price. Hopefully they offer. I am sure the girls would have a ball. I know I did !!!
Good luck, Tamara