How to make a character likable?

I'm writing a "beginners" screenplay right now and I was thinking of
ways I could twist up the ending.

So the main character meets two little kids (both boys) along the way
of his adventures and at the end one dies. I want to make the viewer
really feel saddened by the death and wish it wouldn't have happened,
as well as feel anger towards the person who killed them.
This would really only be the darkest part of the entire thing.

So I know how many people (like myself sometimes) get annoyed with
children so I wanted to break through this and make the sadness
universal, like watching a puppy die or something (sorry, only
comparison I could think of right now). Are there any tips for making
child characters more likable and less annoying with their antics?
Right now I simply have the character who dies as quiet, naive, and
kind of dumb. He always asks the other child to say what he wants to
say.

Anyways, thanks in advance.