There is a whole chapter on Children's Messages in the book, The Art of Teaching the Faith, found on Amazon and Sola Publishing. .
The truth is that the majority of children's object lessons aren't for the kids if the kids are under 8 years old. In the early elementary years, faith takes the form of story or drama, which means the young kids who come up to the front of the church for your object lesson, lamb's lesson, or children's message can't connect the object to your intended concept. It's a higher-order thinking skill. They hear it as two separate ideas or stories. Their little brains just can't connect the two.
Cognitive Development of Kids 3rd Grade and Below (Ages 3-8)
Children in the Narrative Stage of faith development:
Have little ability to think about non-physical entities (God, heaven, etc.) God the Father is a really big dad in the sky. Heaven is a literal place, like the library or a big park in the sky.
Visual symbols do not initially have symbolic meaning -- children must be taught what the cross stands for, for example, and it will stand for the church or Jesus, but not death that pays for sin.
Cannot relate one fact to another, for example, the heart being like the inside of a tootsie pop that was hardened on the outside by sin? No way.
Cannot make generalizations about what happens in a person when the word of God is shared. Super, no way.
Cannot distinguish religious stories from other stories. There is none more or less important. Jonah and Pinocchio are on the same level of truth or make-believe.
The stories of miracles and fairy tales are the same thing in their minds. Miracles happen in fairy tales or many of the movies they watch all the time.
The older they get, the more they can determine the difference between reality and fantasy,, but that's different than taking something that sounds like it can't be real (coming back from the dead) and assuming it's real.
What Should You Do?
If the majority of the kids who come up during your children's lesson are younger than 8 or 9, there are steps you can take to tailor the message to them. If the message is really something for the adults in an easy-to-understand wrapper, then please give the kids candy when you're done for being willing to come up front. Just because they don't understand the larger concept of love, sin, or that Jesus loved them enough to die for their sin, they can be introduced to the words, and the older elementary kids are very interested in the meanings of words. The younger ones will most likely equate sin to being mean or doing bad things and cannot transfer that to other actions, and certainly not thoughts. They understand what they experience. Remember that REPETITION IS YOUR FRIEND!
Be aware that you don't turn children's messages into law about their behavior. "Jesus loves you when you're good and do the right thing." "Jesus isn't happy when you..." Good or bad, Jesus ALWAYS loves them.
Use age-appropriate vocabulary.
Use poems, riddles, and songs that the kids can repeat and learn.
Use props, pictures, voices, puppets, and/or volunteer actors to help tell stories.
Have repeat phrases or a central truth that the kids can say throughout the story or message.
If you have an abstract moral, they won't get it, but they can answer simple questions like, "What happened when I...?" "What happened when (the puppet) hit the other one?" "What would happen if you...?" The information needs to be relevant to their very egocentric experiences.
Show pictures with stories. Kids use pictures to help them understand the story. They can also answer questions about the picture. "What's happening in this picture?" "Is the man happy or sad?"