As the class picks each choice for the song, refer often to the type of song the kids decided to write. Often, parts will be played or decisions made which can steer the piece away from the intended goal. By referring to the style the students initially chose, you can guide their choices and stay on track. Sometimes it's fun not not stay on track and let the creative flow guide the piece. Then, compare the end product to what had been intended.
Kids love sound effects.
I encourage this as often students end up creating wonderfully programmatic music. For instance, a funky, fast-moving song may turn into a cop show TV theme song with the tasteful inclusion of a siren or car squealing away. As kids are so influenced by TV and movies, often they will want to tell whole stories with the aid of the sound effects they have available. This is an excellent exercise in developing critical listening skills and should be allowed and encouraged, at least at times. Students will find that sometimes they don't have a particular sound effect for what they are trying to say, so they will create an approximation of the sound using what they do have.