Contests tend to have their own set of submission guidelines posted on the website. But if you want to know the general rules it's basically:
Double-spaced lines.
Proper formatting.
Name of contest and entry's title in heading of email.
Address to judge/head of department running contest (do research - not only is it good to know who you're pitching to, research will most likely also turn up all the contest 's rules).
Keep the email sorry and concise - confirm that you ARE writing to enter the contest, how long the entry is, genre (if it's not a genre-specific), and end with your name.
But, really, check the website. I'm pretty sure a magazine as big as Highlights will have a whole page devoted to submission guidelines. Take your time to research your market, same as any other submission.
Dave
Edited to Add: Five seconds. That's all it took for me to find the submission details on their website:
http://www.highlights.com/highlights-fiction-contest
I'm not trying to be snappish, but you really should research your target market, and that includes individual submission guidelines. There are no 100% constant rules, so you be to constantly aware of the different demands.
Best of luck!