"Both dcs are very academically able and I want them to be encouraged and challenged but I've heard of some kids doing 17 GCSE's and part of me thinks that some of them are "mickey mouse" qualifications and put unnecessary presure on the kids"
Jen - To go back to the point you made in your OP.
Unfortunately, not all schools will enable bright academic children to fulfill their potentia. My advice is to take a good look at the sixth form.
How many pupils stay on to the sixth form to do A'levels?
What's the range of A level's offered. So, for example,if a school can't offer a science A'level - then you have to question their GCSE science offering (not enough bright pupils, no specialist science teacher?).
I would pay more attention to A'level results than GCSE results. If a school is mainly focussed on it's GCSE league table position, then it might be tempted to see its bright pupils as 'exam fodder' there to prop-up it's GCSE league table position, at the expense of the pupil's own personal education and development.
An academic child needs to have their education focussed on A'levels and university entrance. At most private schools bright pupils will sit 8 or 9 GCSE's. But they will be prepared well for A level.