Have you considered parking your campervan in the accesible camping ground? It wouldn't be near the kidz field, circus, etc., but it might make it easier in other aspects, maybe?
You need to register with the Accessibility Team and they'll provide you with wristbands that would allow you use interstage shortcuts, disabled toilets, a shuttle bus between some areas and viewing platforms. As far as I know, that would be for the person with the disability and one PA only, but you might want to check with the Access Team to find out what happens when you are a family with other young kids.
Take a look here: https://www.glastonburyfestivals.co.uk/information/access-information/
A lot of information on this .pdf too: https://cdn.glastonburyfestivals.co.uk/wp-content/uploads/2019/10/2020-Access-Information-PDF.pdf