Unless the seedlings are showing signs of dormancy (leaves yellowing/browning
/dying) I wouldn't worry about trying to force them into dormancy yet. You
can grow these plants into summer, but maybe consider keeping the soil a
little drier than usual to reduce any chance of rotting the tubers (rather
unlikely). As soon as they appear to be stopping grow or dying back, stop
watering 'til next autumn.
Alternatively, you could remove water now, which will encourage them to start
dormancy, but if they're growing OK at the moment, why not let them? (There
could be some risk in doing this, as the tubers may not have had time to fully
develop). We did this (accidently) with our D. planchoniis, and they survived
but I wouldn't like to try it again...
P.S. Our whittakeris which are growing in our Cephalotus tray are doing very
well and are now producing flower buds. Our Cephalotus isn't growing well
at all :-(