It starts off showing children at the movies watching the movie we're about to see. Several times during the film, the camera moves back so we can see the children in the theater watching.
At one perilous moment in the movie, the little girl heroine is in trouble and the camera moves back and a little boy in the theater stands up and says, "I'LL HELP YOU!" and climbs into the screen.
It is the 1970 Czech movie "Lucie a zázraky" (a.k.a. "Lucie and the Miracles" and "Lucia und die Wunder"). It is in the list of CCFF films HERE and has been discussed in previous posts.
An original Czech language version of the movie is available on VHS, but only in PAL video format. Please see the Lucie a zazraky thread.
I have a very low resolution English dubbed version of it (captured from a narrow bandwidth Internet video stream). Although the picture and sound quality are very poor, it may be good enough to jog some childhood memories. Please let me know if you would like to see it.
NB the file is too big to email (12MB), but I could upload it to my free web space and send you the link.