Revealing mistake: When Kate is telling the kids that their dad is quitting his job, the boy called Mike looks directly at the camera.
Revealing mistake: When Mark takes his pet frog to the other frog's grave, you can tell that the pet frog is a rubber toy, rather than a live frog. (00:18:55)
Answer: It seems that the fact the family can't take care of themselves is added to part of the humour of the film. 12 children is a lot, and think about all those rows and all that mess.
Hamster ★
It's not like they couldn't take care of themselves. As shown with the breakfast scene at the beginning, they seem to function pretty well when they all get along. Tom and Kate were just usually home more frequently than some other parents. It also didn't help that it was the first time that one of the parents was gone for an extended period and the parent that was left home (Tom) had never had to deal with all the kids on his own. Also, the only time we see complete chaos in the house is the first day that Kate has been gone. After that the kids are more behaved (at home).