Will try and keep this short. Was going to post in AIBU but thought this more appropriate and I'm a wuss
My DP and his ExW are going through a divorce.
We have his 4 children 3 times a week. I get on with them very well and care about them deeply. They seem to love spending time here and adore their dad, who is a brilliant father.
Since we've been together, if we need to go anywhere, we put three in his car and one in mine. We go to their DGM, days out etc. and they always fight as to who will come in my car 
A few weeks ago we were getting ready to leave and the eldest said 'Mum says we're not allowed to go in Sissys car' when DP asked why she said 'In case we have a crash'
???
I was a bit upset, mainly because they all looked so uncomfortable and torn and it shouldn't be down to them to relay that.
DP spoke to them calmly and said that was a bit silly because they are not anymore likely to have a crash in my car than anyone else's. (FWIW, I have passed a specific tactical driving course for my job and have never had anything even close to a crash)
They agreed and we sort of just let it go, they continued to come in my car as and when.
Today the exW called him and cut a long story short she says if they come in my car when their here then she will not let him see them 
Please help.
It's unreasonable right?? I feel like I'm going crazy.
Everytime I think of them being put in this position it makes me so upset, because I know how torn their loyalties must be. Likewise I feel if we bend to this ridiculous whim just to stop her withdrawing contact we will always be at her mercy.
Oh and it's perfectly safe with all the right car seats etc so it's not that.
AIBU to think this is ridiculous? And is there anything we can do??