Here is a good metaphor definition for kids...
A metaphor is a figure of speech that makes a comparison between two unlikely things or ideas. A metaphor says (or implies) that one thing is something else, but it doesn’t use the words like or as the way similes do.
My brother's bedroom is like a garbage dump. (simile)
My love is a red, red, rose. (metaphor)
My love is like a red, red, rose. (simile)
His dog is a ball of fur. (metaphor)
His dog is as fluffy as a ball of fur. (simile)
Similes are often easier to recognize because they have those very helpful identifier words like or as. A metaphor can be more subtle.
Another reason why metaphor can more challenging is it is that an entire poem (or story) is the metaphor. (See the poems below). One of my very favorites is the classic poem, "The Land of Nod" which is a wonderful metaphor for sleep.
O little soldier with the golden helmet,
What are you guarding on my lawn?
You with your yellow beard,
Why do you stand so stiff?
There is only the grass to fight!
In the ocean of my hair,
Many little waves are there;
Make the comb, a little boat,
Over all the billows float;
Sail the rough and tangled tide
Till it's smooth on every side,
Till, like other little girls,
I've a sea of wavy curls!
Life seems so sweet! I don't know why--
Perhaps 'tis just because the sky
Put on tonight to make me glad,
A dress I didn't know she had.
-Mary Carolyn Davies
Sing a song of popcorn
When the snowstorms rage;
Fifty little brown men
Put into a cage.
Shake them till they laugh and leap
Crowding to the top;
Watch them burst their little coats
Pop! Pop! Pop!
Sing a song of popcorn
In the firelight;
Fifty little fairies
Robed in fleecy white.
-Nancy Byrd Turner
From breakfast on through all the day
At home among my friends I stay,
But every night I go abroad
Afar into the land of Nod.
All by myself I have to go,
With none to tell me what to do--
All alone beside the streams
And up the mountain-sides of dreams.
The strangest things are there for me,
Both things to eat and things to see,
And many frightening sights abroad
Till morning in the land of Nod.
Try as I like to find the way,
I never can get back by day,
Nor can remember plain and clear
The curious music that I hear.
-Robert Louis Stevenson
All of these illustrated metaphor examples and many, many more can be found in Mentor Poems for Teaching Metaphors.