It's either from the 80s or 70s and its about some kids that runaway, (may be have a goal) may be two teen-aged kids and two or tree more little. I have a vague memory of it. But have a rare scene fist: A chubby girl that have their first period. The teen-aged girl helped in a pier, but leaves in a police department. When the mother picks up in the police department, slaps him. Thanks in advanced.

The Legend of Billie Jean (1986)
Starring Helen Slater, Christian Slater, Keith Gordon, Yeardley Smth (the outspoken little girl who begins menstruating)

You can read the plot summary here:
The Legend of Billie Jean - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/The_Legend_of_Billie_Jean)

Here's a clip from the movie:
The Legend of Billie Jean - Trailer - YouTube (http://www.youtube.com/watch?v=vKZBZBlB_U4)