Let's see. Dad really wants to pay less child support and to change the schedule to live with the kids in the summer. You really don't want that.
Because he doesn't come to visit once a month as it is, so they can't go instead in the summer. And dad saw them less than that last year, so they can't go now in the summer. And he doesn't have a close relationship with them, so they can't go in the summer. And that he abandoned you and has a new girlfriend, so they can't go in the summer. And he doesn't call once a week so far, so they can't go in the summer. And he cancelled visiting in April and May, so they can't go in the summer. And (unstated) your concern seems to be if they do go in the summer, you might get less money (because your costs go down accordingly).
Regardless of how you've painted him, or how awful he actually is in the real world (and true, he doesn't sound wonderful), you come across as punitive and controlling. "He didn't do things the right way in the past, so I won't allow this new way. And it might cost me money (legally, justifiably -- but still, I want the money!)"
I mean, probably he'll back out of the summer because that seems to be his pattern, right? And so it'll be a non issue. But why OPPOSE this latest chance at establishing a better connection between dad and kids? Because you can and have that power as CP? Because he's not "good" enough? Perhaps this is the one way that might work for him, allows him to avoid you, and the long drive and so on, and cancels your concerns his lack of familiarity with their daily school lives and whatnot. It seems it MIGHT work out better than any other schedule tried so far. To disallow it because he's been a prick in the past seems vengeful, not well intentioned.
I'm not saying it's easy to be the bigger person. I've frequently been a prick. But I do know I should try to be nice, and do what I can to foster a great relationship between kids and BM.
One thing that frustrates posters about boards like this is most everyone tends to have a well-honed story that generates sympathy in real life, which portrays themselves as the good, innocent, victims of an awful ex. But in the harsh light of a message board, that stance is easily questionable and frequently sketchy.