You might not be familiar with the area/type of festival. I am - I grew up in London, am raising my own children here, and work in the arts.
These kind of outdoor, free, very public, summery things are often not aimed specifically at children - they are, rather, 'family friendly', so that you can meet up with friends, bring children or not, relax, drink, eat, maybe join in, and enjoy yourself. We were (are? not sure) planning to go to the Hofesh Schechter weekend to meet adult friends, along with our children.
I did look at the full line-up when it was being promoted a couple of weeks ago and thought that the 'Glory' weekend was a bit out of keeping with the general Southbank, family, summer vibe, and would not have considered going to it, but I never thought the content would be as bad as this.
Maybe this 'joke' isn't extreme for a drag act in a club - I don't know, it's been a while since I've seen one - but that just indicates how much drag in general is really working in an area, and with a discourse, that isn't at all fit to involve children in.