Young seedlings at that stage don't need supplements and may be harmed so it isn't generally recommended. It all depends on the nutrient levels in your particular product (kelp extracts come with all sorts of varying nutrient levels), how well you dilute it, the potting mix you are using and its water retention capabilities. In other words, there is no simple answer given all the variables.

Once they have 2 sets of true leaves a VERY diluted (1/8-1/4 strength) supplement may be of assistance but how much again depends on the nutrient levels in the supplement you are using. N stimulates top growth which, without care, can quickly out-strip the root growth needed to support it. With seedlings it is root development that is vital, not top growth.

Hope this helps.

Dave

Thanks Dave. That is helpful. I think I'll hold off on the kelp for now.