we took the ds's last year (on tesco vouchers), they were 4.5 and 17mo. Both loved it, and I would imagine would enjoy it at 6/7. Had to drag 4.5yo away at the end of the day. No idea on Culdrose. Goonhilly was interesting aswell (also Tesco).
Flambards and Culdrose air day are great for 6 and 7yo. Loads to keep them occupied. We take our 4 and 3yo to flambards about 2-3 times a week and they love it, but they are also too small for a lot of the bigger rides there.
Air day is a great day out, loads of displays, helicopters to sit in, etc. I think there is a website with the itinery for Air day, let me have a search, otherwise I have it in the paper.