Firstly I''m no expert but ... You have every right to 2 nights or more providing they are loved, fed, warm, safe, get enough sleep, get up for school on time etc. and providing you are not some axe wielding nutter.
More importantly, what do the kids want. This can be tricky. To start with my youngest didn''t want to go to dads. The ex''s way of dealing with it was to bundle him in the car hysterical and then wheel spin off. Mine was a bit different, over time the youngest came round and now is excited about his weekends with Daddy. You have to get to a point where it is not about money, or rights, or hurt pride of feelings of vulnerability or rejection.
IT IS ABOUT THE KIDS, WHAT IS RIGHT FOR THEM.
Can you talk to your ex sensibly? If so do so, when the kids are not there. If you can''t get some light
mediation or objective 3rd party to help you. Kids are not a currency. Do you want the kids 2 nights in order to make your outgoings more affordable of because you really want to see them. If it is the latter surely that is more important. Offer her, short term same money but you want them for 2 nights. If she says yes well you will then know which side of the fence she is on. If she agrees, do this for a few months, take lots of photos, keep lots of receipts, keep a diary. Then go back to the CSA with your evidence and see what they say.