The small task done independently can feel larger than the adult imagines.

Doing something alone for the first time is not a small event to the person doing it. Paying for the bread, posting the letter, walking to the corner: from outside it takes a minute, and from inside it is the first proof that the world can be handled without anyone standing next to you. That feeling is the whole point, and it is easily deflated by a well-meant correction.

Let the job be done imperfectly rather than stepping in. Praise the fact that it happened rather than the standard, and then find the next slightly larger thing to hand over.