I would say yes... although, a good quality, balanced fertilizer would be the best. I use MiracleGro liquid for houseplants, diluted... and I water with this mixture more often. Others use different kinds of plant food, such as fish emulsion, etc... Hippeastrum bulbs are heavy feeders, and definitely require some sort of food after expending their energy to bloom, as they do.

As far as your babies are concerned... I suppose that different climates and care will produce different results... for example... I planted seeds on May 16th of this year, and they are just getting second and third leaves now. Some are a few inches tall, while others are still quite tiny, and they're all growing outdoors in full sun. I think there are a lot of variables to the question of seeding size, and I don't think there is a concrete answer. Hippeastrum seedlings are slow growers, though, so my opinion is that yours are doing fine.