Of course first born children shouldn't be entitled to more than the others, this is a ridiculous concept. However, each parent should be treating all their children equally.
In this case, the two youngest children have benefited from the situation. Their mother is insuring they get the same in terms of financial support but they now also get to stay home with dad.
The youngest doesn't get any benefits from her dad, him being able to look after her during the day is no benefit since she's at school. However, she potentially might be losing something financially, that is unless her mother makes up the difference.
So indeed, no different to OP, EXCEPT, and that's the big difference, it is natural within a couple that you would support each other, and make up when one can't keep up their responsibilities. It is not natural for an ex to have to pick up the responsibility of an ex, and that's the crux of the issue here.
OP sees it that she had to work more hours, why shouldn't the mother do the same. Well, Anon, you do it because you love your husband, want to make his life better, and are pleased that your children have a better lifestyle than full-time childcare. His ex care couldn't care less about helping him and having to work harder to do so.
You are comparing yourself to her as a mother when what sets you apart is not how you behave as a mother but how you behave towards the man who can't or won't fulfill his responsibilities.
The whole thing has grown totally out of proportion. If it is to believe that indeed, he/you have almost 50% caring responsibilities (which I don't believe) and that his unemployment status is only temporary (which I'm not convinced of either), then of course she is making a big fuss over nothing, ie. £100 lost over say 3 months.
However, what might have got her to become 'abusive' is that she counts the days very differently to you, might believe that your OH lost his job through his own fault, and/or that he has no intention at all to get another job until your youngest is at school. If that is the case, and when she said something, she was told that if she's not happy, she can just have her DD with you full-time, then yes, I can see how anger would have taken over.
The only thing that can be hoped is that your OH will indeed do everything to get another FT job and that the maintenance will resume shortly, and in the meantime, the ex will manage to make up the difference so that their joint DD doesn't have to do without.