Baby and toddler music classes are a growing trend; in fact where I live there are there are at least 5 different classes in a 5 mile radius, plus another two that offer music-with-baby-signs. I've experienced a few different music classes (although not the signing ones) and the variation can be surprising.
There are two types of these classes; those that are franchised from a big organisation, and those that are created by one or a few individuals. The accompaniment will usually be a CD and the class theme specified so that wherever in the country you attend a class you should be singing the same songs. But this is not to say that the classes will be the same. I have experienced three different teachers from one well-known baby music class franchise, and their individual styles came through in all the classes for better or worse!
One ran her classes with calm efficiency. A new song was added each week, another one dropped, songs were accompanied with appropriate props. The next was the complete opposite. This one adored kids and music, but the combination of an cavernous, echoing church hall, a heavy foreign accent and a reluctance to sing the songs to us before we were expected to join in all meant that none of the class actually found out the words. I fact, it was only when I took my younger one to classes two years later that I finally found what the words were meant to be.
The approach of these classes is usually very professional, as the teachers are interviewed, trained and required to stick to the lesson plans provided. However the plinky-plonky backing music begins to grate by your second child, and you start to think that you don't really need help to sing 'The Wheels on the Bus'.
Classes run by enthusiastic individuals can be much more interesting and varied. They will either make their own pre-recorded backing tracks or better still work unaccompanied or play instruments such as the piano or guitar. They have the freedom to provide a more interesting selection of music and may include a craft session to boot.
You may be lucky enough to have a class nearby where you can experience the teaching methods developed by composer Zoltan Kodaly. He suggested for children that songs should be easy to learn and use a limited range of notes so as not to strain the young voice. It can take a while to get used to the more subdued melodies if you are used to the plastic cheeriness of most music aimed at young children, however the classes are much more interesting, with more variety of songs and defiantly no 'Wheels on the Bus'.
Most classes seem to include a session where the children have a chance to join in with percussion instruments, or you will if your child is too young. If you are choosing a class, I recommend that you try to find an age-specific class, particularly for babies and toddlers who may be overwhelmed in a room with 3 and 4 year olds rushing about and banging drums.
Do children gain anything from these classes? As well as the opportunity to meet other children and you to meet other parents, I personally believe that early exposure to as many different types of music as possible creates the possibility for a lifelong and varied love of music.