The openings that work best are the ones you've studied the most and lead to he middlegame positions you understand the best. Blitz is basically all the chess (patterns and evaluations) you've studied and played enough to have stored in long term memory. To prepare for blitz play blitz games... but if nothing is in long term memory there's really no point in trying to prepare.
The anti-chess advice would be to have them play very aggressive, suicidal moves in hopes that their opponent will burn time on the clock trying to figure out the complications and they'll win on time. Teach them dubious gambit openings you'd never get away with in a "real" game. This is what you'd do for players who have no experience but you still want to increase their chance of winning in blitz.
This is also the obviously faster way to get a short term increase in blitz skill, so new players are drawn towards these bad habits... thus pfren's response.
I run the chess club at our local elementary school. Several of the kids have registered for a blitz tournament (5 mins per side, with no time delay or increment) to be played the evening before a regular Swiss-style weekend tournament. I am looking for sources of advice I can give the kids to help prepare them for their blitz games. What sorts of openings work best? Are there recommended strategies for managing their time (especially as time is running out)? Etc. Any suggestions or referrals to other resources would be most welcome. Thank you.