Many years ago I made myself a batch of musical symbol cards from a cut-up poster board. I drew and printed the signs and words by hand, and believe me, they are not fancy! But I've used them more often than any visual aids I have.
Since repeating a song many times, over many weeks, is the best way to lodge it firmly in a child's mind, contually varying the repetitions keeps things interesting. It's also fun to teach children musical language, and they enjoy learning strange symbols like fermatas and crescendos, and the cool Italian words that go with them.
After briefly explaining what each word or symbol means, I like to hold up different cards as we sing a song, and change them without warning so everyone pays attention. It's fun to sing staccato notes, then change to legato in the middle of the phrase. Flip the crescendo (getting louder) sign upside down, and it becomes a diminuendo (getting softer). Hold up the fermata sign and everyone keeps singing the note they were on right when you flashed the sign.
The cards in the illustration above are just a few of the many cards you can make. Accelerando and ritardando (speeding up and slowing down) are really fun. You'll be able to think of others.
It's a good idea to put the signs or words on both sides of each card, if your pianist sits behind you, so he/she will be able to see the instructions and quickly make changes as you sing. I like to hide the cards on a music stand so they can't be seen until I raise them up.
Have you ever tried incorporating musical symbols or language into your primary music? Please share your thoughts or experiences!