But surely they're little minds and nervous systems and everything else will be fucked up by dealing with his poor behaviour by themselves every weekend or whatever he demands?
The difference is that right now their nervous system and brain never get to experience safety. They have no respite.
So never get to come out of high-alert, hypervigilance, threat status.
That's what causes the most damage.
Of course the ideal is that they never have to deal with a shit dad. We would all want that for you and for them.
But if 5 days a week or 12 days a fortnight they experience safety all day and all night - and always know when they are in less stable environments that they are returning to safety soon - then they get to return to a normal safe state.
Their brains and nervous systems get to develop healthily. Their bodies aren't perpetually flooded with adrenaline and cortisol.
The normalisation of his behaviour is separate but again, when they are immersed in it 24/7 they have no perspective on how life should be. You could explain normal behaviour but they wouldn't be able to understand because it would make no sense in the world they know.
If they are only exposed to it every other week they can see it is not normal and get to return to normality afterwards. Like having a bad day at school or work - it is a blip that feels rubbish for a short period but passes.
The point where they will be able to make their own decision about whether to spend time there will also arrive faster than it feels now.
When the majority of their time is spent in a safe home feeling safe and not having to walk on eggshells, and you are free to provide age appropriate explanations on his behaviour and why it's not ok then they get to heal from any damage they've experienced so far. And you can genuinely mitigate for the little bit of shitty behaviour they do witness.
Yes, it still has some effect if their dad is a dick every other weekend, but if the majority of their life is spent experiencing safety then no, it doesn't "fuck them up" in the same way as permanently being on high alert 24/7 because home is so stressful and unpredictable.
Surely you can see the difference in effect if a body is flooded with adrenaline and cortisol 24/7 with no break, compared to only occasionally? You know how it affects you to live like this, right?
It may not have come across that way, but I really don't underestimate the turmoil you are experiencing. I get that this is a horrible position to be in, and I get that you want to protect your children. All I am trying to say is that despite your best intentions and best efforts it is not possible to protect a child living like this (nobody could) - but you can if you leave.
Most of the time in life we aren't presented with choosing between a terrible path and a perfect path, we get two non-ideal paths and have to choose the least bad option.
Leaving and allowing them to have a safe home - a safe haven as a pp put it - and honest information is transformative once you get past the adjustment period and is the least bad option by a very wide mark. It will be tough for you to see that while you're still immersed in this yourself, but it is the better option.
The strength of my reply is because I fear that you will stay on the terrible path indefinitely delaying and hoping a perfect path appears if you wait long enough. Unfortunately I don't think there will be one, but there is a much better, happier path available (albeit not as ideal as we would wish).
I wish you all the best and hope that you are able to find your way out and forward.