
Gohan has a visible tear in his clothing and also remarks how extraordinarly powerful Son Goku is upon meeting him for the first time; he likely didn't expect the child to be so powerful and he probably held back at first until he had to use his expertise knowledge in Martial Arts to defeat him.
Everyone for the longest believed that Goku as a baby had a power-level of 2 or so, and that is really plausible, actually. Being born with a very low powerlevel as an infant/newborn makes sense, but as he grows, his power-level has to grow as well, does it not? Despite being only three, Goku looks like he could be about five or six. Gohan doesn't look the least worried about Goku and his capabilities, but the scuff marks on his face and the tearing in his clothing suggests that Goku's power-level at this time is far greater than even the most athletic of Earthlings.
I believe if he really were to conquer Earth, Goku would be able to do it seamlessly as long as he relied solely on the full moon; it's apparent that in base form, at this stage, he isn't even remotely powerful enough to take on master artists like Son Gohan, Roshi, King Chappa, Nam, Korin, Tsuru-sennin, Tao Pai, Pai, etc.
How strong do you suppose Goku could be at this age? Son Gohan trained and raised Goku for another seven years before he died just two years before the beginning of Dragon Ball. Goku did bump his head after while and took Gohan's training, and proved strong enough to break a big ass rock and lift a car at the age of 12, nine years later after landing on Earth.
And while Martial Artists like Gohan and Roshi knew the absolute definition of holding back and could fight without using their full power (key examples would be Goku, Roshi, and Tien holding back in the 22nd Tenkaichi Budokai), there are no signs of Son Gohan and others being an absolute master of ki control, so I doubt Son Gohan would have been able to lower his ki/power-level all the way to the single digits (if you still believe Goku was that weak at the time).
I believe Goku as a toddler surpassed the power-level of "10" he was given at the beginning of the manga; it doesn't make sense when you take this "retcon" into consideration as well. How strong do you suppose Goku was when he first landed on Earth?