Teach the symbols for dynamics to your class. Explain what each one means. The letter "p" is for piano, which means soft, and "f" is for forte, which means loud. You can also introduce "mf" for mezzo forte (medium loud) and "mp" for mezzo piano (medium soft). More advanced students can also learn "pp" for pianissimo, or really soft, and "ff" for fortissimo, or very loud.
Listen to some classical works with varying dynamics. Give each student a set of flashcards with the dynamics symbols written on them. Have students hold up the flash card for the dynamic they think they are hearing in the music.
Play the conductor game. Hand out rhythm instruments to each child, and have one student be the conductor. The "conductor" raises and lowers his arms to indicate to the rest of the students to play loudly or softly, respectively.
Hold up a flashcard of a dynamic marking and ask your students to play the rhythm instruments at the dynamic level shown on the card.
Show the markings for crescendos -- gradually getting louder -- and decrescendos, or diminuendos, which mean to gradually get softer. Play crescendos and diminuendos on drums by gradually getting louder or softer.