What might my SIL mean?

My SIL just posted photos of her two young grandsons on FB looking happy while swinging at a playground. The caption reads, “We all love a sick day at the park.”

I asked, “I don’t understand. Is one of the boys sick?” Her response was, “actually both are sick. Preschools do other not so nice things besides teaching.”

I have no idea how to interpret this response. She is highly sensitive, so I am asking the collective to help me understand what the circumstances she is alluding to might be before responding. Or, should I just drop it?