The mother’s death
Easy to blame the kids for opening the door, but they’re just kids. And they haven’t been impressed on the seriousness of the situation.
The mother should sat down and solemnly told them what was going on. When the kids asked about the banging, instead of just saying, “It’s not Dad” say “It’s someone who has already been infected, and we cannot take the risk of letting those people in”
To just say, “it’s not Dad” was provoking childish curiosity and inviting the kids to be disobey.