The short answer is that there's no minimum age. Children must be mentally mature enough to take part in the session, must listen carefully to instructions to ensure that they are safe, and must have long enough legs to be able to be set up safely on the bike.
All child riders must ride with a parent or other responsible adult who is responsible for their safety - and when you take out a membership, your children ride for free.
We do this because it's good for children to see their parents enjoying exercise - and they're more likely to build healthy habits as a result. Also, children see a class and they want to join in - so as long as they're mature enough to take part, they're very welcome to do so. Fox is happy to promote healthy families.