OP here, just finished this trip.
Am updating the thread as requested :)
Hope it will be helpful to anyone else wanting a Spanish language summer school.
Thank you SO much to all the contributors who took the time to share advice and thoughts. It was genuinely really useful. When I was planning I read and re-read everyone's thoughts - it was a huge help. Shout-outs in particular to @pixie5121 and @Pafmo.
We chose Don Quixote Spanish Camp for two weeks in Castelldefels (a beach town near Barcelona).
DC (aged 13 and 10) did Day Camp, which is Spanish lessons for three hours in the morning and then activities for the rest of the day.
It cost €850 per kid for the fortnight.
I stayed in a nearby apartment and dropped them off and collected them. They slept in the apartment with me.
It was a great holiday and we all enjoyed it. I’m glad we came and I’d do it again.
I’d score the actual Spanish progress at a 7/10 though.
Castelldefels is a really nice place to be based. People have been friendly and kind, except for the taxi driver from the airport, who over-charged us.
Although I’m a huge supporter of smaller languages, I did hesitate over choosing a Catalan-speaking region for our Spanish course. I needn’t have worried about this at all. Of course Catalan is everywhere on the streets and on signs but DC haven’t found it confusing and it hasn’t affected their Spanish acquisition.
Don Quixote / Don Quijote are the same company as Enforex. They are called Enforex here - I haven’t seen any mention of the DQ brand at all.
What I like about the camp is the staff. There are loads and loads of them, all very friendly, keen to help, cheerful, smiley, full of energy, endlessly patient and enthusiastic.
I wouldn't say the camp was perfect but I'd definitely do it again. Happy to answer any questions.
I'm keen for another language-learning trip next time - might look out for a language exchange.
Huge thanks to everyone for their help.