Well, looking at booking our flights out to China soon and unfortunately it looks like the cheapest option will probably be with a different airline than our preview trip so building up miles on one frequent flyer programme won't happen. This time it looks like a BMI/Air China combo via LHR rather than KLM via Amsterdam. Shame, as Schipol is a nicer airport and we'll probably have a long walk from Terminal 1 to Terminal 3.
However, I thought I'd sign up the family with the BMI frequent flyer programme. But you have to be at least 18 to join. However, it turns out that Star Alliance programmes are interchangable so I checked out Air China's (minimum age 12) and Lufthansa's Miles and More (minimum age 2). So Miles and More it is. How odd. FlyingBlue (Air France/KLM/SkyTeam's) one seems much more coherent.