The seedling phase is crucial but also the easiest. As long as they're not growing in a bone-dry substrate or drowning from too much water (actually, asphyxiating from too little oxygen in the root zone), they'll just keep doing their thing.
You don't need fertilizers, as the cotyledons (the "fake" leaves) already contain enough nutrients to see them through the next phase, which is when they'll actually start growing a lot of new leaves and when nutrients should be introduced (just don't overdo it, they don't need much at first).