He's only 35?!
To have got this bad this young must point to a seriously disordered past.
And hoarding tends to get worse and worse as they age, so this really is only the beginning.
What you say about his family and how they treat you is so sad.
The giggling, squealing and grinning when you try to explain how it feels on top of it all would seriously alarm me - it sounds like there is some kind of personality disorder in play.
You cannot continue to inflict this man and his disorder on your children.
However nicely he plays with them, that is superficial. It's a choice of temporary activity.
Deep down, he will harm them.
Think ten, twenty, thirty, forty years on.
The children don't have friends round, because they're embarrassed - and their social development suffers.
They don't have the space they need to play, craft, study because their spaces are full of his shit - and their development suffers.
They observe him behaving like this and acting weirdly to you - and their perspective on relationships suffers.
They seldom come home from college or their own homes because he's turned your nice home into a squalid dump.
Dirt, dust, flies, fleas, rats, maggots, rot, spiders, fire risk, risk to floors and ceilings from the weight. These are all very real hoarding hazards.
The inability to find anything you need when you need it.
Later, they don't care to bring their own children or to be around him because of how disturbing they find his behaviour.
They resent you because you did not take action as the person whose role it was to protect them.
He maybe dies before you ... And you have to deal with all of it, probably alone.
Is that the future you want for you and your children?
It sounds horribly harsh. But you have a CHANCE. A chance to save your future relationship with your children and to return to the nice home you once provided for them.
I'd suggest you photograph everything. Every pile, stash, heap, mess, hoard, cramped living space, the wrong things in the wrong place, things in the children's space.