Stop being such a mug!
His family are bullying you into doing something that places you at risk, your toddler at risk, interferes with your work potentially and is also against the law.
I don’t trust that even if he said he would quarantine with the kids that he would.
So pull the plug completely, refuse the trip in writing telling him why and that should he wish to take this up legally in any way at all that all communication he’s spewed in your direction will be support your position in court.
You don’t owe this bloke a thing, he’s NOT putting your kids first, he’s putting them in an impossible situation
Let hell break loose. Let him go with his new gf and her kids, let them quarantine or not and keep you and your kids out of all of it.
If your kids are upset, so what! Their dad could have arranged to take them somewhere without quarantine but he didn’t. You’re not responsible for him or for his girlfriend or her kids, you’re responsible for keeping the roof over your heads, you’re responsible for the kids and nothing will ever be allowed to jeopardise that.
End of discussion
You have the passports right? Get them out of the house and somewhere safe and send the email ASAP.