Install call recording app, I use Total Recall on Android:
https://play.google.com/store/apps/details?id=com.killermobile.totalrecall
(I don't know what a comparable app is for ios - others may).
Make sure app is set to record both sides of the call (test it first) and set the options to silent (no beeps or "Recording started" announcements)
Call the scumbags and plead with them to drop the case - tell them the impact it will have on future premiums/job prospects, etc - anything to prompt them to say how much more in need of easy cash they are, than you.
Explain that you're concerned that coaching their son to embellish his "injuries" would set him down the wrong path, etc, etc
You don't need to tell them you're recording (at all) - there's no GDPR/Privacy worries for a private phone call (only businesses have a Legal obligation to inform you they're recording).
Once they've admitted clearly they're committed to the fraud, tell them "Well, if that's your choice, there's not a lot I can do about it" and end the call.
DON'T tell them you've got the conversation recorded, don't tell them anything at all.
Save the recording with Time/Date/Scumbags Name and email it to yourself so you've got a record of it.
Then hand it over to your insurance company, with explicit instructions that if they settle anyway (because its potentially cheaper than fighting the claim) you'll inform the FSA and ombudsman.
It's what I'd do, in your position.