I agree that despite your statement that no agreement is in place it is obvious one is in place and you are aware of it. Until or unless you both agree to vary it you should comply, regardless of work schedules.
I see a core problem here for your ex. The amount of times you seem to be contacting her to discuss something agreed and non urgent is intense over a short period of time and there is no compelling reason for it.
You aren’t together anymore and there is no need for you to discuss child care and your work schedules. When she works, how she spends her free time and the arrangements she makes for child care are not your business. She isn’t interested in your work schedule or your days off work.
What this boils down to is you have a free day and want to take the children. But your agreement doesn’t allow for this as this is time she has the children. She will have made arrangements or plans if she is working. Many people will stick to these arrangements because it provides routine and structure.
The only reason you need to contact her is to advise her when you can’t have the children and won’t be picking them up.
Her boundaries are clear. She doesn’t want to interact with you. I don’t know why that is. But you seem intent on ignoring the boundary and creating a confrontation involving the children.
Stick to the arrangement you know you have. If you want to change it then speak to a lawyer about starting proceedings. If you keep on contacting your ex in the way you are doing there is a huge risk she will take legal action to limit contact with you.