If they are in filthy rich potting mix then they don't need anything else. Many of us grow seedlings with no additional fertilizer until transplanted to their final growing place unless that transplanting is going to be long-delayed for some reason.

And if you were going to feed them then the standard recommendation is a mild liquid fertilizer diluted to no more than 1/4 to 1/2 strength.

Over-feeding young seedlings leads to heavy top growth that cannot be supported by the slower root development.

From the FAQ here on Fertilizing Seedlings - Seedlings are babies. You don't give babies steak and potatoes, right? :)

Dave

Here is a link that might be useful: FAQ - Fertilizing seedlings