Let a child enjoy being good at something without immediately raising the target.

The moment a child does something well, the natural adult response is to see potential and reach for the harder version. Offered immediately, that teaches them arrival is never permitted and success is only a door onto more demand. Competence needs a little while to be enjoyed before it becomes a platform for the next thing. The pleasure of being good at something is worth having on its own.

When a child is pleased with what they can now do, stay there with them. Ask to see it again, or let the easy piece be played a few more times. The next target will still exist next month.