No, I just think you're perhaps being a bit naive in thinking that because children are older, they don't need you, or that you should just look after the younger one.
Where I live, all the soft plays are very strict on not letting the older ones in the younger sections. Therefore, if I want to be there for my eldest, I have to divide my time.
Remember there is a big difference between a 5 year old and a 10 year old. I have seen some real bullying going on in the bigger sections of these soft play places, we can't just assume that because the children are older, they can be left to themselves. Older children do get hit too. They do also scream and cry too. It is not the prerogative of younger children.
I find that I end up dividing my time between both my daughters - keeping an eye on both. Yes that does mean that sometimes I leave my younger child playing happily whilst I go to check on my elder daughter. Its the only practical thing to do.
And not, I'm not saying it is okay for the younger child to get hit... Its not okay for any child to get hit and yes, I do want to be there when my child gets hit. But I want to be there for both of them, whether its the eldest or the youngest. But the reality is when you have two children of different ages, and they're not allowed in the same areas - then you have to split your time. You do need to be realistic about this.
And as you suggest going elsewhere - what activities do you suggest to fill a summer holidays that will keep children of three different ages all occupied and in the same place at the same time, yet remain affordable...?