The gayest laddie of all the group;
Line 19: "At last came one of the merry troop,"
Line 20: "The gayest laddie of all the group;"
" At last came one of the merry troop, "
(Finally, one boy from the cheerful group comes forward.)
" The gayest laddie of all the group; "
(The boy who is most cheerful in the group is the one who stops.)
Overall meaning: One cheerful boy separates himself from the group and comes forward.