Why would he be ashamed of killing a child who was wielding (probably poorly but still) a shardblade and pointing it at him? We learn earlier in the chapter that Dalinar's plate is pretty wrecked between the multiple falls and then the fight with Thanalan, he's also lost at least two pieces of plate altogether. With the Thrill pushing him, plus basic sense of self-preservation, of course he'd attack/kill a shard wielder. All that aside, he was also a vastly capable warrior facing down a 6-yo so yes, its quite possible he was able to disarm the boy and let he and his mother live if they went into hiding.