Not quite the same, but my husband's ex moved away. Initially, it was with their child. But due to her extreme resistance to my husband visiting, he now has custody. And despite the ex's flowery assertions that her son's the most important thing in the world to her and that she bases all her decisions on what's best for him (including moving him away from his Dad), she has remained 2,500 miles away, even while her child has lived here for the last 2.5 years. She's also in a new serious relationship and is rumored to be getting remarried, so I assume she's out there for good. My husband volunteered to waive child support so she'd have money to come visit monthly, like he used to do (which was a huge financial sacrifice for him - she sure as heck did not volunteer to reduce C/S to facilitate him visiting!). But she only comes here twice a year.
Realistically, how could someone feel other than betrayed and outraged by such choices on the part of a parent? Before people flame me for being judgmental: Yes, I can imagine scenarios in which extreme circumstances give a parent no other choice than separation from their child. But regarding my husband's ex - and the OP's - the NCP has consciously failed to prioritize their children over other things they want for themselves - proximity to their own childhood family and friends; a preferred lifestyle; a preferred job, etc. That's crummy! And while most people get used to whatever life they have (i.e., the OP and her kids will get used to the ex/Dad not being around), that core disbelief and outrage over the NCP's choice not to be there doesn't go away!
I love my step-son. I'm happy to do motherly things for him. And honestly, it's easier on me to have no competition for the maternal role, most of the year. It's simpler for me to just treat my step-son like one of my own kids, because he's not spending time with his Mom EOW, so I don't have to worry about stepping on her toes. It's also nice for my husband and me to be so far from the oppressive, relentless conflict with his ex. REGARDLESS, every time I chaperone one of my step-son's field trips, plan his birthday festivities, take him to the doctor, cut his hair, hem his clothes, meet his new teacher, attend his parent-teacher conferences with my husband, comfort and counsel him through conflicts and misunderstandings with friends; when I'm there, cheering, at every single one of his sports games; when I attended his 5th grade graduation... in the back of my mind there is incredulity and anger that his mother chooses not to be here, to participate in these things.
My step-son (who's 11 now) has a pretty level-headed approach to it all. He's close to his mom and loves her and feels sure she loves him. He also wishes she'd move back here. In fact, he thinks it was a "bad idea" for her to have moved away in the first place. But he's very certain that she won't move back, because "she's happier where she is". He knows it's important to her to live near HER parents and childhood friends and that she prefers life by the coast, in a bigger, more cosmopolitan city where she has a fancier, more impressive lifestyle than she did here. He's calm and matter-of-fact about this, as though he thinks it's right that HE should be the one, being happy that SHE'S happy, not the other way around.
Expecting an 11-year-old child to understand that these things are more important to his mother than being a daily part of HIS life; expecting him to understand that she spends money on all these showy things, but doesn't spend more money coming to visit him makes. me. want. to. puke!
OP, I have no doubt you'll feel similarly. There's no way around it. But your kids have you. They'll adjust and be OK. It's rotten that they have to be, but kids are resilient. You will have to be, too.
You can tell him that you hope he changes his mind, and that the kids need him to be a parent every day, not just 3 weeks in the summer. As the mom in the situation it also stinks cause you will be the only parent and will have to shoulder all the responsibility. 
