One of the easiest ways to build a melody is using "Triad-Melody Technique".  Triads offer multiple combinations of organizing the three chord-notes through repeats, inversions and rhythmic patterns.  One of the most well-known examples are the big band era #1 hit  "In the Mood" (composed by Manone, Razaf and Garland; released 1939 and made famous by the Glen Miller orchestra).  Notice and listen, starting measure 9, how the triad is simply arpeggiated and rhythmically displaced across the 4 beat measure.

 In "Music Composition for Teens - first course"  chapter 9 is devoted to creating melodies using triads.  Check out that publication (on amazon or this site) with multiple lessons, photocopiable pages, listening ideas, performance ideas and fun lessons including an Etude, Piano melody,Boogie, Piano Piece, Strathspey, Fanfare, Handelian Recitative, Jazz-A-Tazz, Try Triads, and an Easy Listening Song!  

Word painting reflects the literal meaning of music in sound.  

For example:  waterfall = cascading descending motifs, mountain = ascending and descending scale passages, whirlwind = repeating, quick, semitone motifs, thunder rumbling = low pitched, mezzo piano, repeating semitone motifs, and so forth.  

This is an easy entry point into composition, especially for little ones!
How?​

• Select a poem or verse, e.g.: Hickory Dickory Dock, the mouse ran up the clock, the clock struck one, the mouse ran down, Hickory Dickory Dock.  
• Find the words that could suggest a particular sound.  So, from the above verse, it would be "ran up", "struck one", "ran down" and one could insert something like "tick, tock" for "clock".
• Now, explore and experiment with various sounds on the keyboard, percussion or xylophones to make a quickly ascending sound for "ran up" (ascending scale or glissando), likewise for "ran down", "struck one" would require a loud drum beat, or a low, forte note on the piano, etc.

Options

• Perform as group composition by using narrative (narrator or small speech choir), instrumentalists adding sound effects, singing choir, etc.

OR

• Perform as solo composition by composing melodic parts for the verse/poem and adding in the sound effects as body percussion or piano sound effects, e.g. glissandi, and/or body percussion.

See my new composition for piano and body percussion, "Hickory Dickory for Many Mice" on www.sheetmusicplus.com