who do toi think those two boys in the theme song that are playing in the river?

I don't know if they're meant to be anyone in specific ou not, but knowing the details of this show, I wouldn't be surprised if I'm missing something.

I think the main thing to take away from the image of them is the bateau they're playing with - presumably, the ferry Wirt, Greg, and Beatrice go on to get to Adelaide's.