Is this arrangement via a court order? If so, I would suggest she uses her days to take them to her parents. It's kind of you to use your time to take them to her parents, but maybe suggest she uses her days instead?
Tricky one this.. The arrangement with her parents having child on your days only works if she is being reasonable, respectful and amicable. The fact that she wants CMS as child is at her parents on your days makes her unreasonable and therefore you are not denying child by asking her to let her parents have child on her days. No one is being denied anything except obviously her on her days but this is only due to her reasonableness.
As you have had this arrangement in place for some time I'd quickly get the matter to court (via mediation) to get a CAO in place.
However you may want to explain to your ex that not only will a CAO mean that no CMS is payable but also that it makes the arrangement quite strict and that you will not allow child to go to her parents on your days.
Call her bluff she may back down. I see it as her denying child time with grandparents due to her own unreasonable CMS demand rather than anything you have done as after all the arrangement has worked thus far.
If you get an order and then allow child to go to her parents then that sets a precedent and opens up opportunities for neither party to stick to the order. In which case you might aswell not bother getting an order..
Does she need the cms money? If things are otherwise amicable and you can afford it I wouldn't go to court over it... she might decide to restrict access and it will fracture an otherwise successful relationship, including with her parents.
It will cost you money to do the court action anyway and potentially many thousands if you need a barrister.
Also even if you succeed in 50:50 CAO if she's receiving child benefit she may very well say she is the main caregiver and the CMS will believe her, meaning you'll then have to take it to tribunal which itself can take over a year to go to hearing.
how old is your child? do you share the school holidays with ex? looks like your seeing your child very often. court action will likely make her more hostile and could result in you having less time with child while hearings go on. really the child should be seeing maternal grandparents in your ex's time.
only way to challenge that would be going through courts. you could argue that it's unfair that your child is seeing maternal grandparents in your own time. you would have to aim for court order to state that there is equal day-to-day care of child between both parents. but there is no guarantee you will get that, and could still carry on paying maintenance.
just be wary with your application. it would not look good on you if court thinks you made application to get out of paying maintenance. keep it strictly focused on kids and spending more time with them. ask for half of school holidays, permission to take them on holidays abroad etc.