Since you are working toward having the students sing in a round, it’s important that they have a solid sense of the beat, so that when it’s time to split up, they’ll be able t hold their own. One easy way to get students to practice the steady beat without boring them, is pulling out rhythm instruments and giving them plenty of opportunities to play their favorite. This might mean creating different stations using hand drums, claves, djembes, castanets, and so on. Then, have students play the steady beat while you sing, and give them the chance to switch stations each time you repeat the song. As they grow more comfortable with the melody, encourage them to begin singing along. You might even consider telling them to keep a beat with found sounds, such as a pencil on a music stand or taking off their shoes and slapping them together. Beware of the chaos that might ensue with this one!
I would encourage you to split this lesson into several different class times. The melody can be tricky, and it’s best to give them plenty of practice singing in unison before attempting a round.When you think they’re ready for the round, split them into three groups and have each group stand in a circle facing the center. Give clear starting signals for each group and try your best to not sing along. If it all falls apart, this may be a clue that they haven’t practiced in unison enough, and you’ll need to move back a step so they can strengthen their foundation.
Once your students complete the round successfully, bask in the glow of their accomplishment and celebrate with them. One of my joys in teaching was watching students in awe of their own abilities after singing in a round. It was always an amazing experience for them, and one to be shared with their teacher!