In my opinion, it is absolutely being interrupted. So anytime I am not literally with them, she can come in and take over? This isn't an occasional, once a month thing. Either her or her parents do this all the time.
So even though there is no right of first refusal in the decree, she can just show up during school? How is school different than her going out for an hour and leaving the kids with someone. Can I come by, throw out their dinner, and bring them a whole bunch of candy during their dinner time if she is not physically with them?
(not trying to be snippy, I'm just trying to understand how this is okay and how her taking away my parenting influence during days I am supposed to have them is okay)
Let me flip this around.
At work, I can change my schedule around so that I can be at lunch with them on "her" days. Not that I would EVER do this. But from your logic, it's okay. Let's also pretend that she actually makes them lunches. (Before I filled up their school account with money for buying milk (and they use it to buy lunch on her 2 school days), I was dropping a lunch at school for them every morning, including her days because if I did not, she would not take any responsibility in feeding them or taking care of keeping the school account up to date)
Anyway, what if I show up every monday and tuesday, throw her food in the garbage and bring unhealthy crap for them to eat.
edit: Also, I could probably get a note from the doctor if I absolutely had to and took it to court. They have documented nutrition and growing problems.

