I don't believe there is anything to indicate that Elinor is irritated by children in general. It's just that the Middleton children run wild at Barton Park, and so when she is around them, she cannot think of quiet and tame children with abhorrence. Lucy professes she cannot bear children who are quiet and tame, which, of course, is just as sincere as other things she asserts.

Just because a person doesn't want to put up with children who kick each other, throw things out the window, tear at adult guests' clothing, wreck people's coiffures, untie the sashes of others' clothing, steal the embroidery/crafting tools of their adult guests, throw temper tantrums to get treats, etc., doesn't mean the person is irritated by any and all children in general. Lady M. calls it 'monkey tricks', but her kids are spoiled brats. Why would anyone put up with this from someone else's kids? Anyone who professes NOT to be irritated by this obviously has an ulterior motive for doing so.