One thing that it is of paramount importance when considering music lessons for a young child is not to bring under 7's prematurely into their heads, into intellectual experiences. Little ones have a natural feel for rhythm - and rhythm, whether that which orders ones' day or that which moves through music, has health enhancing benefits. And, further, little ones learn best by imitation - nothing is easier than teaching a young child a song or any movement based on a melodious, rhythmic form. Another point is that the young child has a different consciousness from us adults and is best approached via pictorial, imaginative language. (By the way, parents might consider how much easier it is to "sing" their children into doing what needs to be done rather than constantly talking to them and asking them things - I assure you all - it works like a dream!).
Anyway....bearing all this in mind, if a music program uses imitation - the teacher simply sings or plays an instrument and the children follow, if it is based on healthy rhythmic participation (no lessons on notation or such) and if it's done imaginatively (no technical explanations) then perhaps a parent might want to enroll their child in such lessons - though I honestly can see no compelling reasons for starting formal lessons (however relaxed) before a child's 6th or 7th birthday - and in Waldorf schools individual music lessons on instruments such as the violin or piano are not recommended before the child is around 9 years of age.
Speaking of pianos, one of the reasons piano lessons are - well, sort of "frowned on" for young children - in Waldorf circles is that it IS a very "intellectual" and mechanical instrument. The recorder is favored because by playing it, the child herself becomes an instrument - the music arises from within and passes through her and out the recorder (or pentatonic flute). It is also a very therapeutic instrument because by using the breathing, the child can strengthen and bring harmony to her heart/lung area - and with so much asthma these days, this is very important. Also, the heart/lung area is the seat of the rhythmic area. In playing recorder one is also working with a group - harmonizing, blending in, hearing one's way into communication with others which is, of course, an extremely important life lesson for every child. At home, it might just be the parent and the child playing recorder together - but they can still play together. The piano is, for the most part, a solitary instrument and does not require cooperation with another person.
For all you parents of piano playing children who are reading this, if piano is what your child plays, well, there you go! I would definitely recommend you look also into the recorder, though, for the above reasons. And why don't YOU learn it and teach your child! You don't have to be a musical genius to play simple songs on the recorder! (go to the kindergarten page on the Christopherus website to hear and see music for songs suitable to sing with little ones and play on the recorder with slightly older ones).
I should say in general one wouldn't want to start any sort of musical instrument with a child before she is 7 (or 6 1/2), before first grade. Any activity which requires close attention and needs a measure of accuracy is a waking up activity - and one wants to preserve the dreaminess of the young child for as long as possible. Singing and musical games do not have this "waking up" effect - indeed, if done in the "mood of the fifth" as recommended by Waldorf early years teachers, it will help preserve this state of consciousness until the child herself matures enough to move on.
Back to choosing a music program or teacher, some Suzuki teachers are excellent as the method usually begins by training the ear before progressing onto reading music (again, this is absolutely fundamental to a Waldorf approach - body first - always!! and always work via the senses when you can!). However, some seem to be very driven and perfectionist in their way of teaching - I'd find out as much as possible about the particular teacher first.
One last note....many of us pursue activities - whether it is dance, soccer, bassoon or tae kwan do - with our children at a very young age because we are worried that if we don't "get in there early" then we will miss the boat and our child's opportunities for pleasures and experiences we ourselves missed, are over. This simply is not true! Yes, children learn the most and the easiest when they are very small - but the question we must always ask ourselves is what SHOULD they be learning during those tender years? I would say their most important lessons have to do with learning to be human, learning compassion and a sense of awe and rightness in the world. Gentle non directed experiences, surrounded by Mama singing songs, time in nature and plenty of time - unhurried time - to play and BE - those are the vital experiences for little ones. In this way they make strong foundations for future learning - learning which continues to come easily because so many of the stresses which play havoc on their behavior and health simply are not issues because their early years have been so slow, so BORING by our standards! A love of music is the birthright of all human beings - it will not go away if unattended in the early years (by which I mean formal lessons). Think for a moment of tribal peoples - tiny children do not receive formal instruction in music - but they are surrounded by it, live with it, play with it - and the ones who have a gift later become musicians and the ones who don't participate and enjoy as they are able. There is no fear of missing out!