I took DS to a music and movement class from when he was about 6 months old. He loved it and we continued going when DD was born when he was 18 months old (there is even a video of me dancing with him when she is 10 days old!).
After a while he got bored of it, ran around, didn't want to sing or dance so we stopped after a few months (4 or 5). Plus it was an extra stress to timetable it with DD's morning sleep.
We picked it up again when DD was 18 months and she loves it but DS is now at playschool. There are no siblings I can think of over 3 as most seem to be at playschool (I think we start earlier than NZ so this may be a factor to consider). I think most mothers enjoy it as one-to-one time with their 'baby'.
I have a number of friends who come with 2 children most with a similar age gap or less. The teacher always offers to have a child on her legs for the bouncy ones but most mothers (there are a few fathers) have both children on their legs. The instruments range from shakers which the babies suck to xylophones for the older ones. The main difficulty seems to be dancing with both children when both want to be held.
I'm now pregnant and finding it hassly for the movement bit as DD (28 months) won't dance by herself but likes being whizzed around. She loves the singing and the instrument bit but not the movement and I'm starting to get a bit fed up with it - with her rather than the class itself!
The class is about 35 minutes long with 10 minutes singing, 10 minutes singing with instruments, 5 minutes movement (standing like a tree, on tip toes, etc.), 5 minutes with a CD - flamenco music, country dancing, etc. and then 3 quiet, sleeping songs. It's a drop-in class which I think is great with small children and illnesses, holidays, sleeping, etc.
Gosh, HTH!