When I was faced with giving a birthday party for my grandson, when he turned 9 years old, I started looking around at what other parents were doing. Mostly, they gave sleep-overs.
Our home is not large enough to accommodate 10 rowdy boys overnight. We don't have a pool. Our yard is not large, etc. etc.
There were several options: There is a "sports arena" with sporting gear: batting cages, basketball hoops, etc., which supervised games. One can rent it for about $15 per child. Includes refreshments. One could rent space where a "magic show" could be viewed, complete with refreshments, and the "motor speedway" was waaaaay too expensive.
So, we settled on taking 10 boys to an indoor "rock climbing" place. We paid about $12 per boy for them to climb, with supervision. We provided the refreshments.
It was probably much more expensive than the parties I had when I was his age, but I guess times have changed, and birthday parties have definitely been upgraded in the past 50 years.
I hear you. Since my divorce, the pool parties are out, so The Sportsplex, similar to what you describe, will probably make some money off me, or I may do the local cruise line around the islands in my town. Either way, it's more cash than i have ever spent before.