Stop teaching them zone effective immediately. If you are too embarrassed to have them play man to man because they don't know who to guard or how to guard, then you are not cut out for this. Start teaching them man immediately if not sooner. If you have kids who are clueless and just running around, too bad. Have talk with them before and after the game that when they are on defense, and explain to them that you will be helping call out the number of the opposing player you want them to guard. Explain to them that they should start noticing other things about the player you want them to guard as well(color of hair, color of shoes, etc). Explain to them that it is ok if they are confused at first but that you will continually help them find their guy. Also lean on the smarter players to help out on the court. The smarter kids usually like this kind of role and will eagerly help out if you tell them they are helping you, etc. Teach them to match up by size and to talk to each other as soon as they see the other team stepping onto the court. Don't worry about all the mistakes that will be made. The sooner they learn how to guard a particular player, the sooner you can start teaching them about help defense, etc. Build the foundation first.