We had a daytime wedding and no kids. We had a very small guest list--under 50 people. I only invited two cousins who are both substatially older than me and have always been more like uncles. Had I invited ALL the cousins and their respective spouses, it would have sent my invitation list to well over 75, something we couldn't afford. Had it been cousins, spouses, and children, it would have more than doubled our list.
The other reason I didn't invite them is because many would have had a hissy fit when their kids weren't invited. One of my cousins would have started WWII if her kids weren't invited, and another of my cousins has a history of bringing her WILD CHILD of a daughter even if she isn't invited. I just didn't want the headaches, so I didn't invite any of them.
Unless your cousin ever specifically told you that your children would be invited, I'm not sure why you would have asumed it, even if his mother mentioned it. SHe's not the one making the guest list And there is a big difference between a teenager and ten year old. It's quite possible that they have an age cut for a number of reasons--financial, brats on the brides side of the family, who knows.
The way I look at it, it's the bride and groom's event. You had your own wedding and could choose how you wanted it. If you don't like the way they are doing things, send a card and gift but just stay home.
Anne