They were, but Vader didn't know this; he thought that Padme died, and so did the child (or children in this case). How is that Vader grew so much stronger in the Force in such a short period of time, with absolutely no training?
Also, if Vader was so strong in the Force, and he could sense anyone's location, why couldn't he sense Leia's location as well? I'll tell you why, because they hadn't formed a bond like he had with Luke, and in fact, the only person whom Vader could sense this accurately, was only Luke. This doesn't make him more force-sensitive than the Emperor. It was only until Vader searched Luke's feelings that he even became aware of who Leia was. Doesn't sound like he was so incredibly in tune with the Force after all, does it?
Palpatine was never able to sense Luke's location. After the original Death Star was destroyed, word spread across the Empire that a rebel pilot named Skywalker was responsible. Actually, even Vader knew his name was Skywalker. In the beginning of Empire, Vader is told that they found a trace of something on Hoth, to which Vader replies, "That's it, the rebels are there. That is the system, and I'm sure Skywalker is with them." This scene takes place before he spoke to the Emperor.
Only in the middle of the film does this conversation take place...
Darth Vader: What is thy bidding, my master?
Emperor: There is a great disturbance in the Force.
Darth Vader: I have felt it.
Emperor: We have a new enemy, the young Rebel who destroyed the Death Star. I have no doubt this boy is the offspring of Anakin Skywalker.
Darth Vader: How is that possible?
Emperor: Search your feelings, Lord Vader. You will know it to be true. He could destroy us.
Darth Vader: He's just a boy. Obi-Wan can no longer help him.
Emperor: The Force is strong with him. The son of Skywalker must not become a Jedi.
Darth Vader: If he could be turned, he will become a powerful ally.
Emperor: Yes. He would be a great asset. Can it be done?
Darth Vader: He will join us or die, my master.
Is this a case of poor writing or what? So Vader knows that a rebel who is strong with the force destroyed the Death Star, a rebel named "Skywalker", but can't come to the conclusion that perhaps it may be his offspring? Anyway, that's an entirely different conversation. The point is that Palpatine just told Vader that he believed that this rebel named Skywalker was the son of Anakin Skywalker. So what exactly did he sense? Seems like he just put 2 and 2 together.