so, if OP is claiming 2 CB and 2 UC child elements, and she has 50/50, she will likely lose 1 CB and 1 UC child element? Even though her ex can't claim UC?
Yes. If someone is claiming 2CB and has 50/50 then, if the other parent claims the CB they will likely lose 1.
However, in this case you should keep in mind that her ex was happily allowing the OP to claim 2 CB and paying her £300 a month and only claimed the CB when the Op went to CMS after hearing he'd had a payrise.