Woooooah can we not demonize the couple please? I'll share what's recently happened to me and maybe it will help you see it from their perspective a bit:
My house was channel 5 documentary levels of rank 3 months ago. I hadn't done housework of any kind in over a year, my kitchen stank of dog wee and rotting food all the time and I hadn't hung my clothes up in the wardrobe ever. They lived in the washing baskets. You couldn't cook in my kitchen because the sides were covered in "stuff" and there wasn't a clean surface in the whole house. If I loaded and unloaded the dishwasher once a week it was a busy week.
I hated people coming round to my house, my Mum worst of all and she generally hated being there because of the mess. One day, she came in and went to the loo and while she was in there she bleached my toilet and cleaned my sink. I threw her out I was so angry and didn't let her back into the house until about 3 weeks ago (more on that in a sec).
The reason for all this: I had been living with undiagnosed GAD and depression for 2 years. I work long hours and I'd essentially "burnt out". It got so bad that eventually I started having panic attacks and that's when I went to my GP. Long story short, 6 weeks ago I was feeling so much better thanks to fluoxitine and CBT that I decided to take a week off work and CLEAN my house
. I asked for help on the MN housekeeping board on how to start and they were utterly, utterly brilliant. I've name changed since then but the "Thankyou" thread is mine :) 36 binbags full of crap (7 car loads), 5 days and a whoooole load of bleach later and my house was completely clean.
My house has been totally spotless ever since and so has my brain
I invited my mum in for a cup of tea about 3 weeks ago and she cried she was so happy it was sorted. She now comes over pretty much every week for a cup of tea and a chin wag 
My point is this: No one enjoys living in a filthy house. If you'd asked me back then why my house was dirty I'd have told you "there's no time to clean it" because of my work hours and because that's genuinely how i felt. I would get in from work at 6 and completely zombie for the rest of the evening because I was so low. Once the meds and CBT kicked in, I found hours and hours and hours of time I didn't know was there!
Their house could well be a reflection of what the insides of their heads look like right now, so tread carefully OP, don't judge them and take the house as a symptom of a bigger problem.
Of course, if it's unsafe for the children clean the areas you need to, that goes without saying!