It's not really being altered for her benefit if she has to work! If she was planning a shopping trip or a spa day, you might have a point, but the woman has to work. No doubt she is paying more towards her children's lives than her ex is, even if he does pay maintenance.
7am isn't early for children to be getting out of bed.
I think it's a bit pathetic that you say 'it would take the heat out of it if he would say she's a pain in the arse'. Why would it take heat out of it if he got annoyed? Surely logic dictates that him staying calm about it prevents the 'heat'.
They are his children, and if he doesn't mind being flexible, then that's a good thing. It's a required part of being a decent parent.
You sound like you want him to be pissed off and stressed with his ex just to appease your illogical wants. Why do you want him to get annoyed with his co parent and have his children have to deal with parents that can't be flexible and supportive of each other?
I get that you are an important commitment in his life as well, but you don't even come close to the responsibility and commitment he has towards his own young children. But you are coming across as the 'baddie' because you are causing a problem by complaining when the people that matter the most, the children and their two parents, don't have a problem at all.
You are having a go at him over something he can't control. It's not up to him how his ex deals with things, and even if she does make things difficult he still needs to be reliable for his children. The only alternative is that he creates an argument about something he doesn't mind just to keep you happy, and two children have separated and arguing parents.
Your DP is doing the right thing by his children by being flexible with arrangements.