Sorry, I only just stumbled across this.
Jessica, where is the child based? We run an amazing kindergarten course at Junior Guildhall on Saturdays. This consists of both Kodály and Dalcroze approaches - both brilliant ways of developing musicianship at any age and stage.
Although we don't take them before they're 4 (we take Reception and Y1), the course has a waiting list as long as your arm and so if your friends are within striking distance of London, they need to get the child's name down NOW!
At the end of Y1 selected children move from the kindergarten to the String Training Programme, where they have an individual instrumental lesson and an instrumental ensemble as well as continuing their Kodály and Dalcroze work. It is a wonderful musical education for them and by the time they are 11 they are really rounded young musicians. A lot of them go on to study at the main Junior Guildhall.
The KG course runs in a building just opposite the Barbican tube station and children travel a long way quite often to attend - one comes from Wales every week!
Other than that - if you can find good, well-trained Kodály and Dalcroze teachers for young children, you won't do better in terms of general musicianship - so important and this (and any!) age.
Colourstrings runs kindergarten classes (later they learn an instrument) and there are other approaches for young children such as Stringbabies. Suzuki has a lot going for it, too. (A former student of mine, from the age of 4-18, did her Cambridge dissertation on the three approaches with which she had been brought up - Suzuki (piano), Kodály and ABRSM (violin). It made for interesting reading...
Anything more you'd like to know about Kodály or Dalcroze, please just PM me. And, yes, I wrote the Jolly Music series of books which are designed for class music but which could be adapted for individuals or small groups.