I used to tell the English speaking children to listen to the German, look what the other children do and do the same. If it all went pear-shaped, I'd have another go. I don't think the swimmers ever actually listened to what I said first time around.
If your children have swum in a club before, they'll soon pick up the necessary phrases. Aarefisch used to have some pretty good swimmers. Don't know these days though.
Unless things have changed in England, more confusing than training sessions are 'lower level' competitions as they are run quite differently here.