You are a bit ahead of schedule, aren't you?

Totally depends on where he lives. He's not ahead of schedule here in
Zone 9. I already have six flats of seedings well under way.

most of them have their cotyledon leaves & stems almost to the top of
the plastic greenhouse top

I'm not familiar with the Jiffy Greenhouse, but if it has a removable
greenhouse top, it should be removed when the first seedlings sprout.

How long are you leaving the lights on? The plants need around 16 hours
of light from fluorescent fixtures. Make sure you keep the tubes as
close as you can to the seedlings without them touching.

Cereus' suggestion for a fan is a good one. Also, to strenghthen the
seedlings' stems, gently run your hands over the tops of them every
day, lightly brushing them a bit.

-Fleemo