If someone thinks oh I'll not support my own children then that's on them not me
That's the lazy person's way of looking at it. The non payment of child maintenance in endemic in our society. It is supported by all of us when we assume that it's a private/behind closed doors/there must be more to it kind of thing. Few of us would drop a friendship or call out a work colleague who openly declares 'I'm not paying the bitch a penny 'cos she can afford to go out with her friends and drink vodka'. But we should. We absolutely should challenge the thousands of men (usually men) out there who make children women's business. Children are the business of both parents.
Slagging off the ex with comments about her vodka habit, openly declaring you don't pay maintenance or 'she got the house why would I give her anything' needs to become as socially unacceptable as smoking over a baby or drink driving. The non-payment of maintenance wouldn't happen as much as it does if we called men out on it. If we dropped them from our social circles or shunned them from events or told them it simply isn't good enough and behaved coldly towards them. In it's simplest terms, arbitarily and unilaterally deciding your ex can manage isn't good enough. Supporting a man who makes such decisions shouldn't be good enough for any woman either. Sadly for many children out there, there is no end to the number of women happy to be with men who shirk their responsibilities.
Of course the complexity of adding in new children, blending families makes things very difficult. What I can't personally stand is the assumption that because things have changed for you, they also have to change for the ex. I have lost count of the times i have ordered my life around my ex's contact only to have him decide that contact no longer suits his life and it must change. I have never had the opportunity to say no because the long and short is he will simply not pick up the children when he should. So my life is constantly on hold whilst he makes the most of his. The ex in the OP's situation, may well welcome 50/50 for a whole host of reasons and things will work out great for all concerned. Or she may not. But I suspect she'll have to suck it up and she'll have to fill in the financial gap (because there will be one) and the OP and her husband will justify it because 'she gets all the benefits' and because, ultimately, it works for them.
The biggest thing that needs to change for children of separated families is adults accepting that children are a lifelong responsibility and that children should be a parent's first concern when moving into anything new. Unfortunately, we are selfish beings and kid ourselves that if we are happy, so are our children.