Piccolo Daimao wrote:
Just going to throw out an in-universe theory here. Perhaps only Dodoria and Zarbon (or just Zarbon) had their scouters on listening in on Vegeta and Nappa's scouters on Earth, and then they relayed that information to Freeza, although not mentioning Goku because it wasn't that important. When Zarbon later mentioned Goku and Gohan after questioning him about sending the Ginyu Force, Freeza may not have been particularly listening that well because his mind was occupied by Vegeta, the Dragon Balls, Super Saiyan and/or the Ginyu Force coming to end his problems.
Or, as you said, Freeza simply forgot. The most likely reason, though, is just that Toriyama forgot.
During that particular dialog Freeza is talking about how he has a feeling that a powerful Saiyan is emerging.
Zarbon asks how can that be since the only Saiyans left are Vegeta, that one on Earth and his kid.
Freeza basically replies that he should shup up and get Vegeta (who had been defeated by Zarbon but Zarbon had left him there).
After Zarbon leaves Freeza thinks to himself that it would be troublesome if the Saiyans evolved into Super Saiyans.
The way Freeza thinks of the Saiyans, and not just Vegeta, implies that he thinks that there are more left. Otherwise, Vegeta would be the last Saiyan as far as he knew and he only had to worry about him.
Besides that, I don't think he would fail to listen to what Zarbon was saying or would Zarbon know that without Freeza also knowing about it.
For a in universe answer, its more likely that Freeza forgot about Goku and his son when he was fighting Gohan, maybe because he always thought that were on Earth and so the thought that Gohan could be that kid never crossed his mind.
