Both manga are adaptions from an original story. Tom Sawyer is not a direct adaption, but it still share similarities with Watashitachi no Shiawase na Jikan for being a quick, light and very emotional read. Both could also be considered as coming of age.
Hinako Ashihara's one shot "Butterfly Cloud" reminded me of Tom Sawyer because of its plot: both manga follow the adventures of a group of kids who live in an island, and how their lives change when a new, fair-skinned girl arrives.
Tom Sawyer is more intense and tomboyish, while Butterfly Cloud is more romantic.