Hi Lorelai -
I have only just come across your 'thread', so sorry I'm late!
As a primary school TA I taught recorder for ten years, starting in Yr2 (when fingers are big enough to cover the holes!). I also ran a Yr6 keyboard club, and coached Yr1 and Yr2 on percussion, so children could accompany the Christmas production each year.
I assume you already have a keyboard or piano, so I would certainly think the easiest and cheapest thing would be for you to start her off yourself. Our DS started 'playing' on our Yamaha organ when he was a few months old, and had to be held so he didn't fall off the stool. By age 4 or 5 he was starting to play tunes from the Kenneth Baker books. He went on to play recorder, trombone, alto sax (his main instrument) in his secondary school big band, and also in the county jazz band.
So I think the younger a child can be exposed to some form of music making the better, but it should be the child that sets the pace and 'agenda', and not be pressured into taking Grades unless they want to.
I will try and come back in a day or two with more information on how to set about teaching DD yourself, but for now I will just say I think, at first, the PLAYING is more important than learning to read music; and if its an electronic keyboard with built-in 'tuition' features (lights, etc) try to resist using them at first.
Good luck.