I wouldn't have said anything. It's her party and she can invite who she wants. As pp have said, she didn't tell the children, only the parents.
I've never understood this attitude. It's downright childish, frankly. Accepting this comment it at face value, yes, this mother is at liberty to invite who she wants and exclude who she wants. But announcing it loud and clear on a full-group app (rather than discreetly messaging the individual people involved) goes beyond rude, and into 'I'm openly excluding you, so yah boo sucks to you Playground Politics territory'. And people who behave this way have a lot of growing up to do.
@ChunkyMonkey 4321 your message hits exactly the right note, excepting I'd be inclined to leave out the opening disclaimer 'I'm not being funny but ...' . That immediately looks like someone spoiling for a row, and in any case, no disclaimers are necessary. A bald statement of facts will do the job nicely, and I agree with every word you say (and would do, were I a member of this group).
If your message causes said mother to apologise profusely, say it was a genuine oversight and issue the invitation, it most likely was a genuine oversight. If she starts with the 'you're being oversensitive' line, you'll know it was a deliberate passive-aggressive gesture, and can treat her accordingly from now on.
Either way, I wouldn't be worried that the message might give her a few uncomfortable moments. She didn't concern herself about yours.