My ex also tried to flip it all around and called me a gaslighter. It really is a headfuck. It makes me so angry and sad all at the same time.
This is a classic trick out of the abuser's playbook, they always accuse you of all the things they're doing to you as a way of 'getting in first' or 'making you as bad as they are' IN THEIR OWN MINDS.
My ex variously accused me of
- lying
- needing everything to be perfect
- making him feel like he was walking on eggshells
- being cold
- wanting everything my own way
- being a control freak
- treating him as less than human
- speaking to him with complete disrespect
- having an agenda when talking to him
Guess what he was doing to me all the time he was accusing me of this!
The trick is to clearly recognise that this is all about what's going on in HIS MIND and refuse to allow his mental disease to infect YOUR MIND.
You two are separate and different people but IN HIS MIND he is trying to make you the same as him because he can't stand feeling like you are better than him. It's nothing to do with what's actually happening in the real world (which you can see and respond to) it's the mental illness inside his head.
It can take a while to make this an automatic habit, but whenever you get that shock of "OMG he's just accused me of something completely untrue and unfair" you have to immediately take a deep breath and think, "And that's just how his brain works. He has to blame me and try to make me small/bad because deep down he feels terrible about himself and the only way he can cope is by pretending to himself that I'm causing all his problems or even imagined problems."
I expect that any clothes he has at his house for the kids are wrecked and he knows he should buy some new ones but he doesn't have the money or doesn't want to spend his money on kids' clothes.
He's angry that you're doing well and have a new house, so he's lashing out by saying you that you're the one with no decent clothes for the kids so you should buy some. Simple as that. Once you start to recognise that everything he's accusing you of is actually what he's doing, you can emotionally detach from the accusations.
As you know, however, you have to continue to deal with him, so as much of a pain in the ass as it is to deal with, your best approach is to stay calm, polite and de-escalate without necessarily giving any ground.
"I'm sorry you don't like the kids' clothes. Send them back in the same clothes and I will see if any need replacing."
In this reply, you have not actually said there is any merit in his accusation, but you are giving him a way out of having to send the children in the rags he has at home and is embarrassed about (which is why he is lashing out at you). You already know that their clothes don't need replacing, and you have no intention of replacing any but your response will mollify him and he will probably stop being so horrible or bitching about you to the kids.
I know that coming up with a neutral response can feel like a tough pill to swallow, but keep in mind that you're you're not de-escalating this for his sake, but for your own and the children's, because your ultimate aim is to minimise harm to them and stress to you.
It's a management strategy, that's all.