Kids monologues can be whimsical and fun, dramatic and emotional, or anything in between. The important thing is that the piece is well performed and leaves a lasting impression. The material should be age appropriate and something that the child performing the monologue for kids can relate to. Make sure they understand the material they are presenting. Often, smaller children will learn things by rote and not think to question the meaning of a word or phrase. A good coach will discuss the material with the child actor and ensure they have a good understanding of the kids monologue they are presenting.
If you are a parent coaching your child actor, coach them to present the material as if they were saying it for the first time, as though they just thought of something and want to share it. The kids monologue should come across as naturally as if they were telling you a story from something that happened at school that day. If they are preparing for a theater audition, they have to learn how to project their voice while still keeping a natural quality to the kids monologue they are presenting. This takes some practice but is very possible to perform.
A film or TV audition does not require a voice that is projected loudly but the child still needs to be strong enough to be heard. A very tiny voice can come across as timid and unsure. Even if the character they are portraying is shy and quiet, they want to instill a sense of confidence in their ability to play this role. If your child actor has a small voice, coach him or her to take deep breaths using his or her diaphragm. This will help them to learn to find the fullness and richness of their vocal instrument.