Deliberate mistake: The opening camera scene, that's supposed to be shot from Michael's POV as a child, is too high as noted that he's almost face to face with his parents at the end of the scene and then is only mid-chest up to the father when he removes the mask. The POV is also too high as he walks across the dining room and as he approaches his sister upstairs.
Deliberate mistake: In the first scene of the movie, we soon realize that we are looking at a house from someone's point of view who has no trouble looking over a porch railing, about 6 feet above the ground. It's revealed later that he is a little kid, about 4 feet tall. John Carpenter deliberately filmed it that way to exaggerate the impact of his little kid killer reveal. Unfortunately, when the kid is standing outside the house with his parents, you can see he could never have looked over the porch railing.
Answer: There are only two families that hire babysitters in the film, the Doyle and Wallace families. Where they went is never explained, but it could be potentially anywhere. They could have gone on a trip out of town for the night, they don't have to be in Haddonfield.
BaconIsMyBFF
Where did it state this was all night? It could have been a few hours. Remember that in the next one, it is still the same night and stays night through the entire movie.