You HUSBAND... needs to deal with this and tell them something. Whatever that may be.
HE came home with FLEAS, from their home.
Fleas.... can be inside OR outside the home.
So even if your child is outside, she will be prone to or victim to.... their fleas... and bring them home.
AND I would wonder, about BEDBUGS. Since they are so... filthy. They are not only 'messy' as you say.... but I would call them FILTHY.
Feces all over the place. Fleas. I wouldn't be surprised if they have Bedbugs or HEAD LICE, too.
If they can't even change the litter boxes of their pets (10 cats)... THEN, DO they even... change their own bed sheets and towels????
I would wonder.
I mean, they can't even vacuum. Nor clean up animal feces on the floor.
So THEIR OWN PERSONAL HYGIENE.... would also, to me.... be terrible.
Ugh.
If it were me, and per my kids... I would simply NOT allow them to go there. At all.
Because, I would NOT want my own children, CONTAMINATED with:
Fleas
Bacteria
possible Head Lice, or Bedbugs.
Or, parasites and WORMS, from their pets.
AND do they even take their pets, to the Vet... for VACCINATIONS and de-worming????
I would wonder about that too.
Since they seem.... VERY Unsanitary and Filthy. And can't even take care of their pets. So I would think that they don't even do much else... per hygiene and sanitation and personal cleaning, of themselves, or their bedsheets and towels, and etc.
Their Kitchen... is filthy as well.
Garbage overflowing etc.
Old food on the stove. (possible food poisoning).
Pet hair everywhere.
What a filth pile.
No, they are not 'messy'... they are FILTHY.
I would not let my kids over there.
Your son is also allergic to animals too. SO when your daughter comes home... she WILL have pet fur and Dander on her.... and it will affect your son, too.
TELL your Husband, to deal with it.
Don't make excuses.
THINK of all the ramifications.... of this FILTH at their house (not to mention fleas and whatnot), IF you do let your kids go there.
Your SIL... can join Mommy groups and make friends for her Daughter. So she gets socialized... not only depending on you/your kids for that.
That is a Mommy's... job. To get your own child, socialized and have activities.... on your own, too. Not only relying on a relatives kids.... to do ALL of the socialization of her own, child.
Sorry, but I would no way, let my kids over there.
In my city... MANY people who have OVER abundant pets... and feces and filth like that.... HAVE been, complained about to the Police and the Humane Society... and the pets taken away and given to people who ARE responsible and can take care of them.
Your MIL is in "la-la" land as you said... she cannot take care of all those pets, and does not. NO one in that, house does.
It will do NO GOOD, for you to clean their house for them.
Once clean, THEY WILL NOT KEEP IT UP.
AND it WILL go back to how it was.
YOU NEED TO CALL CPS OR the Animal Cruelty or Humane Society.
AND you are not responsible, for entertaining your SIL or her child.
Your SIL's job... is to parent.
Meaning, that SIL gets out and enrolls her child in toddler programs or Mommy groups and makes friends. Like all Mommies, do.
Protect your kids.
If your Husband came home from there covered in Fleas, your child will, TOO.
AND, your son is ALLERGIC to animals.
Your daughter will come home with pet dander and bacteria, on her.
How can you, feel good about that?
Protect your kids, first.
Not making your home a home for fleas and allergens, which will affect your son.
And who knows what else, bacteria, worms, lice, bedbugs etc.
WOULD you want that in YOUR home???
Because, anyone who goes to their home, WILL be transporting it back, to your home. Guaranteed.
Cat feces all over the place and urine stained walls.
Good grief.
Not to mention the rest of the home.