Married or not, one parent earning over 50000ish means no child benefit (it's best to claim in and then pay it back as you then accrue years towards the state pension)
DD1 was earning around 48000 and her husband about the same. They got full child benefit.
DD2 was a stay at home Mum and her partner earned 52000. They didn't get a penny.
A very unfair system that needs an overhaul.
There is another way to look at that situation though. The first family are paying 2 lots of tax, NI, commuting charges and probably childcare, which is ruinously expensive for many people. The second will be paying only once, will have no travel or childcare costs, and it was, presumably, a choice to have a SAHP.
I have no problem with workers contributing to support families on low incomes, but I see no reason for them to be forced to support those who chose to stay at home.
Having said that, I agree that children can arrive in multiples, (or just unexpectedly), and in a rich country like ours, there is no reason why that should push a family who were managing into a situation where they are not. I feel similarly about when people's circumstances change. For that reason, I support CB for all, even if it is only for the first few years (I think there was no reason to keep paying it until the youngest child was 16). I don't support a means test, for all the reasons I usually object to means tests, plus the fact that IMO it is anomalous for families who can afford to have a SAHP to be subsidised by those who can't.