Be carefull not to let them topple over. Or that the fan doesn't blow them over. If the stem gets a kink in it from falling over, it will probibly die. I had that problem once. Get more lights on them. Put a few CFL's on the sides of the seedlings, this way they wont stretch upwards as much to get to the light...
Get a cheap table fan and get some wind blowing on them , it will thicken them up and also put more soil around them to support the stem a bit .
A flimsy stem can also occur due to the plant straining to get to the light. Make sure the light is close enough for them so they dont have to stretch for it.