He didn't even know he had a son or daughter, so how's he supposed to sense them? It's possible he sensed something with Leia, but didn't know what it was. As for Luke and Obi-Wan it could easily be argued that Ben was cloaking them in the force. Especially since Vader did begin to sense Kenobi's presence, but only after he had finished disabling the tractor beam and was trying to lure Vader away from Luke and Leia escaping.