I really do think you need to refer her to SS for the sake of her DC. They will want to help her make changes to improve their life.
My reasons for having this opinion are based on my own experiences with a similar situation.
Years ago, I worked for a charity that helped families with children who we going through difficult times. One family I helped also had this issue.
I was shocked at my first visit as there wasn't a single surface clear of mess/rubbish. There was nowhere to even sit down and the potty was left on the floor (almost overflowing), with the dirty toilet paper strewn around the room, along with dirty nappies from the younger DC, food, wrappers, plates, cutlery, dirty clothes, etc, etc.
The DCs' room was a mattress on the floor, and a cot; the room was filthy and the dirty nappies were just thrown everywhere (must have been several months worth of nappies). Her excuse was that they didn't have a bin upstairs.
My role was to be a listening ear, and to motivate her and support her to make changes to improve their quality of life.
After discussing how to help her, etc, I helped her to make charts, etc, to encourage her to organize her time, and to focus on one or 2 tasks per day, so she didn't feel overwhelmed, and would have more chance of it becoming a long term change.
We would also talk through any problems she was having, and how she was feeling, and if needed/where appropriate I'd signpost her or offer solutions, or just listen.
I suggested putting a carrier bag on the DCs' bedroom door, as she didn't have the money to buy a bin.
This way she could just chuck the nappies in there during the night, and then bring it down every morning to go straight into the kitchen bin, etc.
Originally, I helped her to tidy the whole house in the hope that she wouldn't then be so overwhelmed, as this is what she said she was most struggling with, and would be able to keep on top of it by implementing this new routine.
Suffice to say, it didn't work longterm. She had no interest in keeping it up, even though her DC were obviously happier and safer in the cleaner/tidier house.
It boiled down to the fact that it was easier to let the mess pile up, and in her own words she didn't want to "waste her time cleaning when it wasn't just her making the mess" (both children were under 5 and she was the only adult living there!).
Even though she knew social services were involved, and this had been set up by them in the hope further steps wouldn't need to be taken, she just couldn't sustain the changes.
It was an unsafe environment for me to be in, according to my supervisor, I think just due to the mess and it being very unhygienic with bodily fluids just 'lying' around.
I had disposable gloves, etc, to wear whilst I was there, and helping to clean, etc.
I wasn't really worried about that though, I just felt sorry for them and wanted to help.
After the first visit, in which I stood up the whole time, I took something with me that I could put on a chair if I needed to sit down. I kept it in a washable bag, which I'd leave in the car and I'd wash it all as soon as I got home and then keep it for next time in the clean bag.
I spent over 9 months trying to support her to make changes and improve their life, but I eventually had to admit defeat, and she was referred back to SS, with our 'reports'/paperwork.
[She was aware that this would happen if we felt we weren't making any progress].
Ultimately, by involving SS you'll be ensuring the DC are safe, and you'll be giving her a chance to make changes and improve their lives with professional support.
I realise it's not an easy decision to make though, but it is the right one. 