Belo - how lovely for all of you that your DD is musical!
I'm going to preface my comments with a disclaimer that I have never, ever gone in for organised classes/lessons/activities for the children, on the grounds that I am a firm believer in boredom. They spent the first five years of their lives doing stuff all outside the home (unless you count poking in puddles and such like). So I am not one for pushing them into activities - in fact, I regard them with horror.
All that said: my DS has had a lovely singing voice since he was old enough to sing. When he started school, his school music teacher said she thought he was exceptionally musical, and asked whether we'd thought of letting him have singing lessons. We had vaguely wondered this ourselves, if only because he was (and is) obsessed with cathedrals and had seen a poster advertising choral auditions, which he'd been interested in. She said she knew the top singing teacher in our city, and suggested that we take DS for a trial lesson. DS was coming up for six at the time, and the teacher said he didn't normally take pupils under eight, but he would give him a go. Following the first lesson, he said he would love to teach him despite his age. His age was apparently relevant from the point of view of lung capacity, which - so he said - isn't really well developed enough for solo singing until about eight. However, he said there was still a lot that he could do with a talented child - and he has done precisely that. Quite apart from having a lovely voice, DS (7 in May) is now a superb sight-reader (better than any 10-y-o in his teacher's experience), and is having a very good musical training which suits him. There is nothing he likes better than one-to-one adult attention . He has subsequently taken up piano lessons, which are going equally well.
It has of course occurred to us that this could be an advantage when it comes to schools - both in the short term (choir prep schools have fantastic scholarships) and in the longer term. At the moment we are paying crippling prep school fees, and we need some help to fund at least one of the DCs!
I'm not saying any of the above to boast about my DS (I could write an entire book on all his foul behaviour as well!) - but just by means of encouragement. Any kind of music is such a lovely thing - if they enjoy it! - for them to be able to do. Six is by no means too young for an instrument (I started playing the piano at four, and still love it now...).
Good luck!