How powerful would a child between Ares and Athena be?

Their child would basically just be Mars, father of Rome

alternatively, a lesser member of the mahkai (war gods). While power doesn't *directly* correlate to age/birth order, children of gods are almost always at least a bit weaker than their parents, rather than a more powerful combination of the two.

Probably strong enough for Ares to get one of those, "Your child will surpass you," prophecies and kill it at birth.

That isn't really how that works. Most male demigods are excellent warriors by default. Demigods don't really have "powers" (except for a handful, most of which got their "powers" from third parties). They're just peak humans.

Athena doesn't have sex with anyone, but especially not with Ares.

I’m thinking Hephaestus is a better match for Athena.

A child of Ares and Athena would be a fierce blend of tactical genius and sheer battle prowess.

I second the mars idea since mars is basically ares with a brain and into agriculture and civilization

That child would be powerful AF and rebel by running off to be the gentlest of water nymphs. Then accidentally run the Pacific Ocean 🤣