When using {IsBaby:0b} on a Zombie, Villager Zombie, Husk, Zoglin, Zombiefied Piglin or Drowned it will NOT turn that mob into an adult, however using {IsBaby:1b} WILL make an adult version of the mob turn into it's "baby" form

Comamnds used:

Was successful 

/execute as @e run data merge entity @s {IsBaby:1b}

Was unsuccessful

/execute as @e run data merge entity @s {IsBaby:0b}