This WhatsApp group is another control move.
Your children in a group with a man who behaved like that with a vulnerable woman, and a group you're not in?
He's proved himself to be a manipulative user.
He will be intending to do one of two things (maybe even both):
Telling them lies about you, planting doubts about you in their mind ('I'm worried about your mum's mental health - it was a bit crazy throwing all that stuff out the window, the removal men all thought so ... I'm worried about your safety with her ...')
Or working on their sympathies ('I did a silly thing, now the dog and I have nowhere to go, we've lost our home ... Wouldn't it be nicer if we could all just get along, be a happy family again, be together? Could you have a word?)
I'd be telling them exactly what he did and why your answer is no.
But if you go ahead and allow this, which I strongly advise you dont, telling the truth is the only way to counter the lies he might be telling while your children are under his influence.