Tenchura! Tensei Shitara Slime Datta Ken

This spinoff manga starts from the end of Volume 1 of Light Novel, although its more like a parody.

After an encounter with Shizu-san, Rimuru learns the art of mimicking humans. However, his appearance was somehow changed to that of a three-year-old...! The smaller Rimuru is, the more mischievous and troublesome he becomes! The monsters in the Jura Forest can't help but pamper him!