How do I challenge a friend to a game? (kid)

Friends can be found in the "Connect" tab on the home page. Here you will see a list of all your friends or you can search for new ones using the "Search Friends" area:

Once you see your friend's username, click on the 3 horizontal lines followed by "Add to Friends" and that will send your friend a Friend Request:

Once the Friend Request is sent you should then see a pop-up telling you that your friend must also send you a Friend Request:

Once you've both sent and accepted the Friend Request from each other you should both be listed as Friends on each other's accounts! 

Friends can also be found in Fast Chess through the "Play" tab, Slow Chess & Puzzle Duel.

To find your friend and play them a "Fast Chess" game, click on "Play", "Play vs. Kid" and then the icon on the right that resembles a person. If your friend is online and you're already connected as friends (meaning you both have sent and accepted a friend request from each other), you will see them when you click on the "Friends" tab on the right:

To play Slow Chess (which can last several days) with your Friend, click on "Play", "Slow Chess", click on the Friend's picture and then "Play":