eh Children of hades' necromancy doesn't drain them a lot, it's the same as a child of Poseidon controlling water, or a child of zeus summoning lightning. it's their basic skill so it's not as draining as special abilities.

Controlling Fresh water is harder then controlling salt water for sons of Poseidon, which controlling other liquids would be dangerous and could 'burn' them up if not careful sense it would take a lot of energy.

Trying to control more then they are able will either cause 1. them to 'burn' up from their power, or 2. just pass out if their lucky.. may sleep a few days.

The children of the big three do have more power reserves than the others, but that's also because they have the most abilities as well.

and of course pyro is Hephaestus children but only rare numbers which we have 5.. so still in range for me to make a character so a son of Hephaestus may show up lol.

Now summoning an entire army, without any help of an artifact will drain a child of hades, just like summoning a huge tidal wave would ware out a child of Poseidon. The longer they train the more they can do of course so one point they could do things they didn't think that could do.