Short answer is every chance they get... when they are not running around, eating, chasing each other, bothering you or just watching the world go by.
Your question is a good one. Kids nap when they feel like it. I like to nap after a good meal myself. Even adult goats won't sleep through the night; they sleep, go see what was left in the food dish, grab some hay, annoy another goat. Lather, rinse, repeat. Love my goaties.