It can be tricky seeing and moving tracks. An alternative is to record the sound effects on the same track that was used for the background music.

It's OK to make mistakes -part of the learning is how to cut the mistakes and replace them.

If the children are recording effects with a microphone, they will need to find somewhere quiet to avoid background noise. They could put the mic close to, but not in front of, their mouth, go under the table, in a cupboard etc.