While I think it would be better if Rocks didn't have a child story-wise (because how would the government not know about it), this is an interesting thing to speculate on. Garp doesn't make much sense unless Rocks is a generation older than the other figures from that era, and then it would be weird that he would have such a position in the marines despite this.
If the assumption is that he had a child 38-40 years ago, then quite a few characters fit that age. Notable are the Yonkos Blackbeard and Shanks. These seem to be the popular guesses, likely because of their prominent role in the story, and possible connections they might have. Blackbeard's ship is named the Spear of Xebec, though this may just be a tribute, similar to how the real-life Blackbeard named his ship Queen Anne's Revenge.
Other characters in this age bracket include, but are not limited to: Bastille, Foxy, Buggy, Enel, Caesar Clown, and Iceburg.