Revealing mistake: When Mr. Kubitz arrives at the house and finds Phil with makeup on, a car is driving by in the background. A few seconds go by before the shot goes back to Mr. Kubitz - plenty of time for the car to have driven out of shot. But when Mr. Kubitz is shown at the doorway again, the car is still there, driving along.
Revealing mistake: When all the kids are gathered around on the first day, and Charlie says "We would like you to read the mission statement", the sun tanned kid Dylan mouths his lines.
Revealing mistake: During the first day of daddy day care, when Dylan is screaming after leaving his mom, we see a purple car drive by in the background and a woman walking down the street. Two shots later the exact same thing happens again.
Revealing mistake: At the "Rock For Daddy Day Care", someone turns the sprinkler on and everyone starts running away. But you can see that many of the people run away before the sprinkler goes off.
Revealing mistake: Phil is not actually playing the guitar when he is singing for the kids. He hits different/mismatching chords at multiple points in the movie.
Answer: Because it's a kids movie that adults are supposed to be able to sit through as well. Seeing the horrific diarrhea mess would have too high a gross out value and be a turn off for the audience. It's better to leave it to the imagination than gross out your audience when that's not what you're wanting to do with the movie as a whole.