[Kalimac]

Denethor doesn't strike me as a cowardly type, but you have to admit, he goes to every possible length to avoid direct confrontation. It's odd - he seems very brave, but in a way he's one of the worst cowards in the books, because he simply refuses to do anything except roll over and die. "Well, I'd probably die anyway if I fought, so why not save the effort and die now?" But he does seem rather past the age for fighting (and not, like Theoden, especially inspired) - it would make sense that Faramir and Boromir would take up those duties. And after news of Boromir's death came, Denethor wasn't very rational and not really acting in anyone's best interests, his own or the kingdom's.

Thranduil does not have old age and despair to contend with; also it's not like there's an issue of "What happens to the kingdom if my heir dies" because Thranduil himself is in for the long haul and theoretically could always have another heir. Also there's no indication that Legolas is his only son. (Same thing with Gimli - we never know if Gloin has half-a-dozen children or if Gimli is it). Even if Legolas is his only son - he did send him to Rivendell as a messenger and is tacitly allowing him to gallivant off with a Fellowship which is in much more theoretical danger than the Battle of the Five Armies ever was. If he'll let a son do that, why not let him accompany him to battle?