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 object lessons aren't for kids under 8 years old. If the majority of the kids who come up for your lessons from 3-8 years old, object lessons are doing what you think they are. In the early elementary years, children are in the Narrative stage of faith education. This means that 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. Their little brains can't connect the two. It's a higher-order thinking skill. Until then, they hear it as two separate ideas or stories.
For more detailed information about their development, look at the Narrative Stage chapters in the book.
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 take the opportunity to 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! Here are some quick tips.
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.
The information needs to be relevant to their very egocentric experiences. If you have an abstract moral or lesson, they won't get it, but they can answer simple questions.
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?"