I think for adults who play an instrument themselves it's always easier for a child to start an instrument younger, as the parent and child can 'play' together without it having to be in a formal learnign set up.
Most musical instrument lessons are more formal i.e. teacher/pupil, 30 mins, 1to1 etc etc.
Both my DSs play instruments, but they didn't really start until they were 6/7.
Many instruments require a lot of concentration and or fine motor control.
If I were you, I would do the following:
2-3 years : Children's music groups Jo Jingles/ kindermusic types
Lots of singing/ nursery rhymes at home etc
Percussion instruments at home to play along with - get a sense of rhythm - marakkas (sp?), xylophone, tamborine etc
4-6 : recorder - a most under-rated instrument in my opinion (can't you tell I'm a former/current recorder player )
By learning the recorder she will learn:
- to read music
- to play in a music group
- the fingering similar to other woodwind instruments (flute/saxophone) for the future
THEN if, at 7, she is keen, then I'd consider a 'proper' instrument
Piano - hard but useful, however not a 'group' instrument
Flute - LOADs of them, but popular, portable and versatile
Clarinet - a bit tricky to start with beacuse of the reed (need adult teeth) but versatile as an orchestral and/or jazz instrument
OR you could go for the more unusual - harp? oboe?