I posted on IMDB years ago about this and smeone said it was the Johnston Monster. Netflix did not have the DVD so I could never tell. Now that I have seen the segment here I know this is not it. Here is what I remember, perhaps it was not on the film festival.
I watched this movie between 1975 and 1979 on a Saturday. 4-6 children were trying to capture a monster in a lake. There were some old ruins, like a log cabin. It was not a giant quarry or loche like the film segment for the Johnston Monster or the Giant Eel. It was very wooded and you never get a good look at the monster. The kids fail and at the end one of them says something like they will have to wait until next summer and the monster will be bigger then. One of the other kids says "Yes, but you will be bigger too". I think one of the kids was a girl.