The other way of viewing that you’ve done all the hard work and now he swoops in is that he is now finally doing his share.
You started this because you wanted more money despite being 50/50 you got greedy and now you’re loosing it all.
The children will be able to see that as well. They will be able to see proof that and know they stayed 50/50 which unless super high earner means no maintenance and even then he was paying you £300 when he didn’t have to.
They will be able to be shown you put in a claim to get money you were not entitled to.
They will see that you are doing this to their dad. Dad doesn’t even need to woo them just show facts. This isn’t even about the children for you anymore it’s about the cash otherwise you would have hand up you’ve been an idiot and be doing 50/50 as you were. But no you are fighting for something you’re clearly not entitled too.
It doesn’t matter if he cannot get the child benefit. If he has them for over 50% you can’t have it and if you do have them 50% it should be split one child for each parent.
Now you have your children only 42% of the time he will get awarded both children and you’ll pay him maintenance and he will get child benefit for both.
Was all of this worth it to try and get extra money you were not even entitled too.