They exposed their vulnerable grandparents to the flu.
Close quarters, poor hygiene. It just makes for a hotbed of infection in day-care centers. Preschool kids have a high level of (flu) transmission, and they bring it home.
What this is really showing us is that there potentially is a discretionary part to emergency room visits. We're not talking about gunshot wounds or car accidents but at the level of a really bad cold.
As the game gets more serious or more important, the effect gets stronger in a linear fashion. We didn't see any spikes after the games in visits. What we assume is that people decided to wait.