Well this might not be open for long:
We get lots of kids riding our BMX track regularly. Sometimes parents turn up with their kids who can barely ride and still encourage them to ride the track. I've even seen some kids on bikes with stabilisers taking to he track, being encouraged by their parents. Some of the 'advice' the parents give is so wrong as to be dangerous 'just stop pedalling if you get scared'. I usually butt in at that point and tell the kids to keep pedalling, otherwise they'll end up rolling backwards down a jump. Some parents have zero sense.
When I offer the use of my BMX so the parent can take a spin, they often say there's no way they'd take that sort of risk. I don't know what sort of kit the kid was wearing when he was injured, but if he wasn't wearing a full face lid, long sleeved shirt (and hopefully elbow pads) and gloves, then he shouldn't have been riding. Not sure what kind of bike that is but it's got a two piece crank, what look like plastic pedals and skate park gearing. My guess is that it's not a serious BMX.
Kids like him need proper guidance on proper track use, and proper kit too. His parents need to take a bit more responsibility, just like some of those we get turning up at the track.
I do wonder if the council has placed a sign up at the track saying people ride at their own risk and that they should be wearing proper kit.