The Hurons are a native American nation. In context, it seems that Kipps and his friends have been accustomed to play at being Hurons:
> He set himself to whistle a peculiarly penetrating arrangement of three notes supposed by the boys of the Hastings Academy and himself and Sid Pornick, for no earthly reason whatever, to be the original **Huron** war-cry.
We can guess that the boys have been reading historical fiction set in America, for example the hugely popular _The Last of the Mohicans_ by James Fenimore Cooper, and they have been playing at aspects of Huron life described therein. So the “two accomplished Hurons” are Kipps and Sid.
“Manifestly” means “clearly”; “their” refers to Kipps and Sid; and “line” means “track, course, direction” ( _OED_ ). So the meaning of the passage is that if Kipps and Sid are to play at being Hurons, their best course would be to go down the lane to the sea.