Celia felt that Oswald neglected her in favour of his brother. That reassured William a little, if only for the thought that it was at least possible that she left under own free will and was somewhere that was more or less safe at the moment. Darker possibilities nagged at him - such as the possibility that she'd been out in last night's riots. Still, with no ransom and no sign of a struggle, he'd have to take the information for what it was. In this case, no news was perhaps good news.
"Gwyn...is fascinated with explorers," said William carefully. "Genna has not taken well to the fact that I have less of my time to offer her. If they decided to go on an adventure, it's possible that they got lost."
He felt his heart wrench at the possibilities that lay in store for two little six year old girls who were lost in the wrong area of King's Landing. There were a number of horrible possibilities for Celia as well, but as a grown woman, she had the sense to know danger when she saw it, and to get away as quickly as she was able. The girls were still at that age where children thought they were more or less invincible.