I don't think anything that the blogger observed was really that extreme.

His/Her writing style didn't really strike me either, it came off as very whiney & dramatic. All that considered...
Toddlers can get annoying pretty fast; the constant picking up of their dropped toys, the costant reminders on how to act in each individual situation, etc. etc. and especially because they are constantly testing their limits! I can't imagine having an older child in swimming lessons & having to stay in the bleachers, and keep my toddler entertained! Just shoot me now!
While the blogger was relaxing & people watching while her child (ONLY child, it sounds like) was in swimming lessons, the parents in question behind her were trying to keep their toddlers from doing things like tumbling down the bleachers & annoying the sh*t out of everybody else there. I'm sure it was hot too. All things combined equal a stressful situation.
Dear Blogger,
Lighten Up, and mind your own business.
Thanks!