Seedlings doesn’t need nutrients, they have everything they need in their cotyledons and by the way they don’t have enough roots to assimilate nootz at this stage. Then every horticultural soil is pre-fertilized, this make another reason to not add any more.
But they really need to get some root booster. So, for the moment , just use the Bush Doctor Boomerang and transplant them in bigger pots (1.5 - 2l pots) next week and stick on the Boomerang. Any pre-fertilized soil is good enough for 3-4 weeks without adding any liquid nutrients. If you can’t resist to play the lil chemist , I suggest you to add beneficial bacteria like Trichodermas, Mycorrhizae, and Humic Fulvic acids only at this stage !