Feel a bit stunned by the whole thing, but really aware that there's no point worrying, and they're probably not afflicted with the condition. Right then:
OH has just been diagnosed with mild haemophilia, which means that since it attaches to the X chromosome our DDs are carriers. We obviously need to find out whether they have the disorder as well as just being carriers, but the clotting agent factor 8 is very responsive to stress, i.e. if someone is stressed then the factor 8 reading may show to be within normal range, when in fact it may be very low. DDs are 6 months (teeny tiny veins!) and 21 months, and will befinitely become stressed by having a blood test.
I guess, I just need to find out as much as possible, and sit tight for a few years until there would actually be a possibility of taking a blood test without stressing them out too much.
I'd love to hear from anyone who has been through the same thing, or similar.