Gettingthere, I wholeheartedly agree with you. Your point is clear and taken. I feel for you, I really do. {{{Hugs}}} My marriage break up went the same way and he and IT are in the same boat as your ex by the sounds of it. All is not doom and gloom. OK, I'm remarried but that doesn't necessarily make it alright.
Please don't fret about your children and christmas. I know it's easy to say and I'm not saying that money does not matter. Pop along to the CAB and speak to a professional who will help sort out your debts. They will write to people you owe and arrange some sort of payment plan for you which you can afford. I can recommend them to help.
Were the debts incurred by you and your ex whilst you were still married? IF so then he has half the responsibility. My ex left me with a barrel full but as both our signatures were on loans/mortgage etc, I was lucky enough to meet someone who told me that I was only supporting my h at the time... not my fault. CAB sorted out the other bits and bobs too.
Surely, if your children live with you then you might have a residency order? I know it's good to share the responsibility but at the end of the day you are the prime carer, what right does he have where the children spend christmas? If he is paying so little (which is more than mine is paying) then he can't afford to drag it thro the courts/solicitor etc., can he?
If you are finding the hours of work difficult because you have to work, then try to cut down your hours, benefits might help?
I am no expert in this area, so do look further into your entitlements.
My dh's ex has NEVER worked. (she's one of the other type, - the NOT exhausted type) and TBH with you she is a monster... a devious one. Short as two thick planks, so if she can drain the system, I am sure that a woman of your grasp can find a way. My ex's bit doesn't work either... nor do I! Only 24hr mum, step and wife. scrubber, cook, taxidriver, nurse, counsellor....!! I'm flippin exhausted!! I can only appreciate how you must feel at times.
I do hope that you don't think all steps have a go at BM;s, just those who deserve it!! We can't all be tarred with the same brush.(I hope)
I'm both and I have to say the balancing act is tough. I have fallen many times.
{{{Hugs}}} and good luck.