Geoff,
I have twins as well. When they were 6, they'd "read" picture books or draw at cocerts or festivals, but they'd also ask questions (quietly) about unfamiliar instruments. They started taking lessons (fiddle and guitar) when they were about 7 and started listening more and more. By the time they were 12, they had separated themselves from us at festivals and located all the blues events. They had also played duets with Dad at our local open stage by this time. They are 30 and both playing professionally now. (bass and mandolin, although they're both multi-instrumentalists)
Can't wait to see your kids in 24 years.