Perhaps the kid is from a foreign land that Naruto visited during one of his missions. The kid could be an outcast in his own place because of some unique trait. Naruto, being the kind - hearted person he is, decides to adopt him. It could be that the kid has a different kind of chakra or a strange mark on his body that makes others afraid of him, but Naruto doesn't care.
If the big kid in the story learns to be kind and helpful towards the crippled kid, the moral could be about compassion. That we should be empathetic towards those who are less fortunate or different from us.
If it's a story about the big kid helping the crippled kid, the moral could be about kindness and acceptance. We should be kind to those who are different from us.
Often, Kid Goku is portrayed as the carefree and adventurous type. He's always looking for the next exciting thing to do, whether it's chasing after a strange animal or exploring a new area. He might be a bit naive in some aspects, especially when it comes to things like emotions.