blutz: could you pretend this is not your own family and look at what you have told us as it appears to me, a stranger?
(I know this is hard, as you are worried, and quite naturally so, and obviously you tend to see your son's point of view first and foremost, as he is the one asking for advice. But I do not think the remarks about cutting your DIL some slack was justified.)
Here we have a young couple who had a very stressful time when their babies were born prematurely.
They both work full-time and have with your and other relations help cobbled together child-care.
DIL is depressed and being treated for this.
In your opinion the house is filthy, so you clean while you are there and do the children's washing to the extend you can while minding two small children.
IMO your DIL should not be expected to work full time while she is being treated for depression. Is this something her GP knows she is doing, something she has insisted on, or something she feels is necessary due to their finacial situation?
Your son needs to sit down either on his own or with you and work out how he can discuss his wife's health with her, making it clear that he loves her and is concerned about her.
If that is the case, that is, as he apparently is talking about divorce.
If he is feeling that divorce is the only solution then he needs to get on with it.
If he loves and cares for his wife still, but is scared witless for his children's safety (which I fully understand) and has mentioned divorce in a panic, then he needs to find an alternative fast.
Either he does the housework when he gets home from work, or by getting up an hour earlier in the morning plus using his evenings doing it, or he employs a cleaner.
It is unreasonable to expect you and the others looking after two two-year olds to do housework as well!
Do they have a garden? If so, he fences in an area of it to make a safe playground for the children.
It is stupid to take them to a playground that is insecure for their age-group. Can his father not help him with this at a weekend? A fence, and a gate that the children can neither climb nor open, a sand-pit, a log to use as a horse, and somewhere to play ball is enough to start with. It doesn't need to cost the earth, and if he cannot utilize his lawn for their play-space, a load of wood-chips can provide a fairly safe underlay, at little cost. Later on, he can put up a swing too.
Discuss this with your son. Then sit down and work out how you can, without seeming to criticise her, ask your DIL what help she needs and wants.
I would start by saying, "I am worried about you, and I don't quite know how not to sound like a nagging mother-in-law, or a know-it-all grandma, but I am not either of these things.
You look worn out, what, apart from looking after the children can I do to help?"
Has your DIL a mother, sister, brother or anyone else who could be roped into help?
I hope you, your son AND your daughter-in-law can find a workable solution fast, before one of the children is hurt or your DIL becomes so depressed that she commits suicide or murder!