What is a word that means to condescend in a positive way, such as the way an adult would speak simply to a young child so they could understand

You could call that "speaking on someone's level".

She spoke to the kindergartners on their level.

The adjective debonair may fit.

Definition: urban, gracious and charming in a cheerful, carefree, light-hearted way ; having a good disposition.

Example: My grandfather is the most debonair person you’ll ever meet. He is a very warm-hearted, genial gentleman who showed nothing but kindness to us.

How about the word vouchsafe?

You can use it in place of condescend as instantiated by: He vouchsafed an explanation to the conundrum

vouchsafe