How much of an X is he? And can anyone who has *not* joined Match or is not logged into Match see the pictures with the kids?
If they are recently split, not yet divorced, etc. and him dating could be a factor in all that, I'd probably let her know. And then refused to get any deeper involved (i.e. she can't use my account to stalk him online, etc.).
If they've been split for a long(ish) time and are officially and legally over, than his having a profile on Match is legit and none of her business and irrelevant, really. And I wouldn't mention it. Drama for nothing, IMO. Even with the kids in his pictures.
Unless, of course, the pictures would be inappropriate for even a married couple to post of their kids online (facebook or wherever) and/or come with enough identifying information to make the kids easy to find. In which case, I would probably say something directly to him about it if I thought he was just clueless.
The only way I would mention it to her is if he was someone I was interested in dating. And then, the conversation wouldn't be about him having a Match account or the types of pictures he included in his profile... but about the fact that I was interested in her ex and how she would feel about my attempting to act on that interest.