Nothing.
I hope that doesn’t disappoint you but I teach absolutely nothing. I sing, jump about, and wriggle my bottom. I don’t teach.
Your amazing child won’t be stopped by that! Despite the lack of a teacher they will learn a huge amount, as if by magic.
First there’s language. They’ll be absorbing speech patterns as we sing along to the hello song, and as they get older they start to be able to recognise rhyming words. Singing it again and again helps them practise forming words. When we sing Bubblegum, they start to offer their own ideas and we can develop what they suggest. Do they mean one knee or both knees? And are their hands stuck to their legs or their thighs? Or calves? Vocabulary building like this is fun!
Then there are musical ideas such as loud and quiet, slow and fast. Children soon learn to to feel and keep in time to a beat as we hammer to fix our tractor, or chop and snip in the chop chop choppity chop rhyme (not to mention developing and practising fine motor skills and their jumping/stomping skills).
Socially, of course, we all continue to learn about being together in a group and how to manage the many small, but strong, personalities in the circle. Is it your turn on my lap, or his turn? Can you hold the parachute still until it’s time for everyone to shake it? These are acquired skills!
There’s a huge amount of learning happening in Rhymetime. And absolutely no teaching.