Ace told Rayleigh? There has never been any indication of Rayleigh and Ace meeting, so unlikely.
I never said Rayleigh didn't know Roger was having a son. Of course Roger would tell Rayleigh, Rayleigh just wouldn't know that it's Ace.
And?
No one is saying that Roger would keep the information of him having a child a secret. Roger, however, would have no idea of knowing when and where his child would be born. The only way for Rayleigh to know about Ace is either through Rouge or Garp. Rouge died right after Ace died, so he didn't get the information from her. He's been hiding from the Marines since the crew disbanded, so he's not chummy with Garp. Garp respected Roger, Garp never indicated that this respect extended to his crew as well.
No, it makes perfect sense for the WG to find this out but not Rayleigh. For one, Rayleigh is one man who lives on one island in seclusion. The WG is a government, they are in charge of the world's distribution of information. As Ace grew as a pirate, they reported it, they have a far greater access to information than Ray does, of course they'd be able to put together pieces they have access too and Ray doesn't.