I'll try to keep it as brief as possible without leaving out anything important.
So exh and I divorced two years ago. He moved out and in with a woman he'd just met a fortnight before and her two young children. Our DC (7, 6 and 4) stay with them Saturday evening-Sunday evening so 24 hours a week.
For quite some time now my DC have been complaining that they don't want to go anymore. I have asked them why countless times and mostly they've told me they don't like exH's partner's nine year old son because he bullies them (something they've been telling me basically ever since they started going there.) The bullying ranges from physical attacks such as hitting, kicking, wrestling and elbowing through to telling them horrible things recently such as "I hate your mummy and I'm going to kill her." 
. My DC aren't known to lie fwiw so I completely believe them. My exh has been informed plenty of times by myself and our DC but still it continues.
On top of this all five children are crammed into one small bedroom, my girls sleep on the floor on blankets and DS shares a bed with her nine year old son who he doesn't even get along with. The DC tell me the house is cold, messy and smells. They regularly return home hungry and when I feed them they wolf it down as though they're starving. When I ask what they've eaten, it's minimal.
But worse than that, today they have informed me exh and his partner smack them regularly and hard. They said his partner has also been saying vile things to my DC such as if they misbehave they won't see daddy again, she's going to shave my daughters hair off and she's going to tie them all up and cellotape their mouths.
I am obviously absolutely shocked and appalled by this and can't believe all the time they've been begging me not to go, this is likely the real reason why 
. I don't believe in smacking but regardless, it isn't her place to lay a finger on my children let alone say the disgusting things she has.
I feel as though its abuse and they're certainly suffering bullying in the least by both her and her son. They don't want to go anymore and it's my place as a mother to protect them so don't want to send them. What do I do though? I feel as though if I stop contact he may pursue court action and what would realistically happen there?
It's worth mentioning he doesn't really have the money for courts and I couldn't honestly see him bothering, he's been fairly lax since he left... He's never arsed to do anything with the children, he's honestly never taken them anywhere since he left. By the sounds of it they're largely left to their own devices in the house for the time they're there or shoved in front of a TV in the bedroom...