If we fail to live up to the requirements of the kingdom then we're in trouble. The little one here isn't just a child, but one who's young in the faith. It might include, for example, this man they've just mentioned who's been healing in Jesus' name. It may be that his healing is part of his path to saving faith in Jesus. So what he needs is not to be prohibited but to be introduced to the real person of Jesus.
Salt is a purifying agent.
It combats decay. So what he's saying is to be sure that the decay that's natural in all of us, the tendency to go our own way rather than God's, is overcome. Keep yourselves inwardly pure, particularly in the way you think about other's. That way you'll be at peace with each other. Don't worry about status or importance.