When a police line is put up on a crime scene it clearly means to stay out of that crime scene and if you do happen to cross that police line then you will be arrested and taken to jail.
Then either you can bond out or stay in jail until your court date.
You could then have to serve a few months in jail or pay a fine and the fine depends on the state you're in.
Now if you were walking down the street and your young kid took off and got past the police line and you only crossed the police line to get your kid out then nothing will happen.
Because then you had a good reason to cross the police line but make sure you promptly exit the police line after getting your kid.
That happened to me once when my 2 year old son got loose from my hand and went into the police line area and I had to get him.
I apologized to the police for having to cross that line and nothing bad happened at all nor did I even get a ticket but if I were to just cross that police line for no other reason then I could be arrested and fined.