There are a number of children's writing contests that can be entered. Some of these contests require an entry fee per submission too. I'm glad to say that Highlights isn't one of them.
Each year Highlights runs a theme-based fiction contest, which any adult can enter. They choose three stories each year out of hundreds of entries. I believe they get about 1,200 entries each year so it's a very competitive contest. The other good thing about this contest is that the company reviews these entries also for possible publication in its magazine even if you don't place in the top three. Shortly after the winners are chosen, Highlights provides the next year's theme so that writers can start to think about putting together a new manuscript or revising an existing one to fit this new theme for next year.
I've entered the contest for the last five years. Although I haven't won or drawn interest from them, I continue to enjoy the contest and have built up a portfolio of stories.
So if you're looking to get a manuscript published by Highlights, I encourage you to enter the contest. Their website is http://www.highlights.com/.