Agree with all members of your family never to pass anything that has been left on the stairs for, ‘going up’, without picking it up and taking it to where it belongs. Don’t allow ‘I didn’t know where it was supposed to go, so I left it…”; or “I didn’t see it” be acceptable excuses - if they don’t know - they should ask - only by asking will they learn and if everything has ‘its place’ it won’t take long for them to learn.
Try a reward system with children. Have a jar for each child and for every good or kind thing they do, write it on a Post-It note and fold it up and put it in the jar. You can decide at the end of the week how to reward your child based on the number of Post-It notes are in their jar. You could make it a family ritual, reading each one out aloud - building the child’s self-esteem, developing good habits for their future, at the same time, keeping the house de-cluttered.