The OP is getting rather a hard time here. The father of the children is not stepping up to support them financially. The OP does not have any "spare" money for new coats and trainers - this doesn't mean she can't budget - if every penny is accounted for just to cover day-to-day living, where can she magic the funds for any extra necessities like new (or even 2nd hand) clothes.
Yet the father of these children books holidays and doesn't even feed them when he sees them.
Rather than having a go at the OP and recommending ways she SHOULD be able to financially cope, maybe we should expend our energies on getting the law changed so men HAVE to financially support the children they fathered?
How can the courts allow this - he says he doesn't have any money - do the courts never question how he manages to feed and clothe himself and go on holidays.
In this case, the law really is an ass, and OP I understand your feeling that if he can't be arsed to help put food on the table for his children, does he really care about them at all, or is he seeing them just for appearances.
Yes, I know it is for the benefit of the children that they see their father, but really does it benefit the children having a relationship with someone who would see them go hungry?