It's not entirely clear whether you're talking about all of them being your kids or whether some are grandkids. If they're all your kids, in my opinion, they should all get "the same" gifts, including the in-laws. So if you're going to do a joint $50 gift card for dinner, then all the kids should get $25 presents, for example.
I know that it makes me feel like family (and my husband too, on the flip) to have my birthday acknowledged equally by my MIL. It would make me feel bad if she sent a birthday gift for my husband but not for me. Of course, if she opted out of gifts for grown-ups entirely, that would be different and that would be totally reasonable.
Finally, I can't imagine there being a time when I wouldn't want to buy gifts for my kids, whether they're grown or not. If it's about finding ways to cut down financially I totally get it. Perhaps you find a way to pare down some. But I do think that it would be a little sad to not give anything at all to the kids for big events like Christmas and Birthday, and if you're going to do for one, I think you should do for all.
Just my $.02