To leave the marital home in a mess

It sounds like procrastinated, overwhelm clutter that builds up over the years.
It also sounds like STBX has contribtued by not doing his fair share of household maintainence, and condoning his family filling the house with additional stuff like the prams.

I suspect that after the OP's departure, it could escalate to non-functional levels of clutter and mess, and that's not fair on the DCs

For your DCs' sake, it sounds like it's worth doing a clear out of your things and their outgrown things. They deserve space to enjoy what they're in to now.

Do it by the simplest means possible.

Would a man help declutter and clean a house before moving out?

I don’t understand saying “I need to know it’s nice for them when they are staying there” when it will by the sounds of it be exactly what they’ve always been used to? If you left a nice, tidy and spacious home and your ex then accumulated so much stuff that it made it harder to live there, I’d understand your perspective. But the kids have been living there now in amongst it all so why would you now be worried about them being there?! And if you haven’t been able to sort it all whilst living there, how will you be better able to sort it whilst popping back in from time to time?

Take important paperwork, take one bag of sentimental items for your children, take any photos that aren't digitised. Leave everything else and make your peace with it all going in a skip - that is actually a blessing!

someone who is living in circumstances where they are being controlled by their partner/partner's family?

Do your children know about the split? I mean I think I'd be a bit careful changing what they are used to at their father's too much, I think they'd need some stability while settling to a new life. If you don't drive, how are you moving your stuff out? Don't you think that if your husband sees you decluttering, he might wonder what is going on? I mean we don't know the whole story by a long chalk but it seems to me that the "you made the mess, you fix it" may not apply here.

Tbh hoarders often blame others for giving them things but who on earth would let someone give them buggy after buggy?

Editing after seeing they rent.
They may lose the tenancy if OP backs out, and be hit with fines.

its rented

She hasn’t said she “needs” to leave. If there are DV issues or similar well decluttering is relevant

Otherwise given people are busy washing out yogurt pots, it’s not that much to expect someone not to freely dump a load of stuff that could be used.

I would only do what I could without creating further mess and chaos before I left ,focusing on taking my things , getting rid of any bits of mine I didn't want and helping children sort their bedrooms. If things are fairly friendly you could offer to help afterwards if he wants you to but be wary as he could say you've got rid of stuff he wanted.

To be fair the OP herself says ideally she would want to come back occasionally after the divorce to go through stuff so it doesn’t sound like a DV situation. The mess seems to be mostly of her making and we should all be looking after the planet by not adding even more to landfill. Without further explanation it doesn’t seem an acrimonious split.

Are things so bad you can't ask your ex what he wants to happen? I think if it were my stuff I would feel guilty for leaving the mess for someone else to deal with. If it's a private rental, the landlord could give notice, then your ex would have to sort it all in a short space of time. I'd imagine this could make him more resentful and further strain things, making co parenting harder.

If there's DV or similar involved and you need to get away and cut contact immediately, then I think that's different. You could look at sending a house clearance service to work with him so you don't have to see him.

You both should get on top of it. You say it's a rented property, it needs to be looked after and surely you have landlord inspections? It will make it very hard if for any reason the house can't be rented any longer and they need to move.

Can you not talk to your ex and ask him what he'd like to happen?

I think you’re being i reasonable OP. If you’re taking the children with you then you either take their junk or get rid of it. You are clearly a hoarder and don’t get too excited about your new clean house - it will quickly end up the way this house is - the fact that you can’t be arsed to sort the (YOUR) stuff out now shows your mindset hasn’t changed.

This is the definition of a hoarder

OP, you have admitted that most of the hoarded stuff belongs to you and the children.
Boxes of craft stuff, SIX prams, 100s of old plushies....
And you are implying that you want to go back and fore for things AFTER you have officially moved out???
It sounds to me that you want to keep your new home clutter free, not by streamlining but by leaving your accumulated junk at your now ex husband's house and "popping back and fore" to pick up a few bits as and when, while tutting at the said clutter in your old family home.
So very selfish, OP.

@DaisyBloo can you speak to your ex .
Ask if you can both hire a skip and everyone works through the stuff together .
Say you don’t think it’s fair he sorts himself or loves like that Wheh you have a fresh start
I’d order the skip and go on each room at a time . Give the kids bags for their room and you need to start with paperwork /important keep sakes and your stuff .

Exactly

the OP’s new place will presumably be like this soon enough too

Put the stuff outside, for free notice up and people will take toys, prams etc

but ultimately if his family dumped stuff then leave it to him to create a pleasant home

I can’t understand how people let their houses get in such a mess! 6 prams?! Why? Why wouldn’t you just get rid of stuff as you go along? Are you a hoarder?

He’ll be left in the house so it will be up to him, but your DC will presumably be visiting.
He could pay for a cleaner.

You could also ask on FB if any of the local charity shops would be able to collect a load of toys and bric a brac. Some do in my town. Worth finding out.

Do a big declutter with the kids roped in, says it a big spring clean if none of them know yet about your plans.

Me though I did go back and help sort it once the sale was progressing. He did 90% of the dump runs I admit

Maybe not put any more blocks in the way of a woman needing to leave a relationship.