The Montage Theatre ones are good and informal - they don't allow parents to watch though. They have little taster lesson at the end of the term where you can see what they've been doing, but would be a bit distracting to have all the parents in the class.
We run classes every Saturday in singing, dancing and acting but unfortunately only from age 6 upwards (we do have a few 5.5 year olds). Anyhow please do bear us in mind for later. We're based at Haberdashers Askes Upper School in Telegraph Hill. www.centrestage-arts.co.uk - and we've been doing it for over 10 years. We also do a free trial and are happy to have parents around.