Mission: Yozakura Family is a Shounen Jump manga series about Taiyo, a formerly normal high schooler who marries* into a superpowered spy family.
In Chapter 60, Taiyo's suffering the aftereffects of overusing his new powers. To aid in the recovery, he's given a potion that turns him into a baby physically and mentally.
There's no process, just a "poof", but we do see baby Taiyo sitting in a pile of clothing. Most of the story is about the family members reacting to how cute babies are. No process on the return either, but Taiyo wakes up in an embarrassing position.
*While the marriage is recognized within the family, Taiyo and Mutsumi are in fact too young to be legally married, so they don't have sex.