handling seedlings

I wait until they have two true leaves, from what environment did you move them, e.g. greenhouse, propagator, cold frame etc.?

Sounds like they weren't very sturdy.

are you pricking out to larger containers, or planting outside?
If the latter the RHS suggests 5 true leaves for brassicas

http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=721

You know those little green canes you can buy for pot plants?  I use them to make holes in the compost and then ever so carefully lift a clump of seedlings with a fork (cutlery kind) and gently hold the cotyledons (seed leaves) and tease the seedlings apart by shaking the loose soil off and gently pulling.  Then insert each one into the hole you have made, deeper in the compost if you can, and guiding the roots into the hole with the green cane.  Then gently firm the compost around the seedling with your fingers  or the cane.  I usually then stand the new pots or seed trays in water for a few minutes and leave to drain.  Leave somewhere warm and out of direct sunlight to recover from their ordeal and they should be okay. 

Brassicas do tend to get a bit leggy and seedlng stems are quite vulnerable to breaking - just treat them with kid gloves.  I have been topping up the compost a few days later on my savoy cabbages and purple sprouting, but they are now growing on in pots on a table on the patio, ready for planting out in a few weeks.  Hope that helps. 

be sure seedlings are well watered a few hours befoer disturbing
R