show them the motions out of water first.
keep em in the shallow end so they can always touch the ground with their feet. if they feel the need to spring up out of water in a panic they wont feel so overhwhelmed.

just keep going through the motion with them. practice showing them how to float by supporting them. each time you guys go in the water support them less and less when they feel comfortable.

Remind them that if they panic and splash about wildly that will help them sink. staying clam and focusing (hard as it is for little kids) is going to help their head stay out of the water...the rest should flow!

good luck! It took me a while to get the hand of swimming. I needed to work on my form...I bet your kids will do great!