This fits the prevailing model where the inviter pays for all except souvenirs.
If you had difficulty paying then you would have stopped inviting. Then a few of your children's friends might be asking "Can I come?". Then you can freely ask for their parents to help pay. =QUOTE
I know the way I did things is different than most posters here, but it worked for us and apparently it worked for my boys friends families as well.
I had no "difficulty" paying and no we didnt "stop inviting" and I did "freely" ask the parents to pay. We did what worked well for our family. For the past 13 years we have done day trips almost daily for the entire two and a half month summer vacation. I didnt care to spend our money on the friends admissions when their own families could easily foot the bill. Like I said earlier if there had been a friend who took my kids on occasion to an event and paid the cost and went back and fourth with treating each others kids then that would have been fine, but it has NEVER been like that. Instead it has been me bringing kids to 100's and 100's of outings myself, while the friends parents were at work.
In a lot of responses, I have read that people invited friends to keep their own kids happy and occupied. If that is the reason, then by all means pay for the friend to go, because that is a HUGE benefit to your family. We never included friends for that reason. Was it fun to have friends join us, by all means, but it was also A LOT more work for me as well. I know I have been lucky, my boys have always been best friends and entertain each other, we never really needed to fill up the van with friends for that purpose. However, I was always happy to have friends join us, (I love kids!) even when what could have been a relaxing day at the beach turned into me applying sunscreen repeatedly, supervising multiple bathroom runs and counting heads in the water. I was always thrilled to have lots of kids join us. However, I quickly drew the line at paying for all of their admissions as well. (Not everywhere we went charged an admission fee, lots of places were free, but when admissions were charged, I let the parents know.)
Like I said, the way we have done things is different than others, which is fine, everyone is different and has different needs, like I always say to each their own.
To the OP, like I said before, I would talk to the friends parents for sure! Good luck!!