@ProlongedAffair
as you have been told by several people- one of whom works in family law, others who have a been in your ex-husband’s position (or rather their partner has)- that is not the case. They do get it wrong, not infrequently and do not always understand their own rules.
Secondly, by seeking to (fairly) get CB for one child to accurately reflect the 50:50 custody, he can evidence to CMS that this is the case. He’s seeking to defend himself from your unreasonable demands, starting by establishing that he has equal custody of your children.
From what you have said, you would have been far better to approach him directly to discuss increased support amicably. He may have refused (quite rightly in my view), but at least you’d have had the decency to discuss it with him. Instead, you went for the nuclear option out of greed and did not properly investigate what the consequences could be.
Now, you have damaged your amicable co-parenting relationship with your ex-husband and may well comf out financially no better off. As he can evidence 50:50 custody and financial support, it is almost certain you will lose one set of CB. You are unlikely to get much, if anything, by way of child maintenance- especially if he can show that he already pays more than 50% of costs, has at least 50% custody (and if he can show more than 50% over last 3-6 months, then you might actually end up paying him) and does his share of taking to dentist/dr etc. (which you have said he does). I think this is case of “be careful what you wish for”.
FWIW, in your ex-husband’s position I would be doing exactly as he is doing- defending himself from you as you have shown you can’t be trusted to act fairly and honestly. I would probably go down the legal route of ensuring shared custody to prevent you from unilaterally changing custody arrangements for financial gain. I hope you don’t do this, as it would be a terrible thing to do that to your children.