@Pancakesandwich Right now I haven't seen anything allowing to do it easily. Adoptable children seem to be made following a specific sim template. It doesn't seem impossible to mod, but very difficult.
Until a solution exists, use the following cheats to apply a life state to an adopted toddler or older child (while the adopted child is the controlled sim) :
- alien : traits.equip_trait occultalien
- vampire : traits.equip_trait occultvampire
These life state traits can be removed the same way. The cheat syntax is the same, just replace "equip" with "remove". So you can remove the trait safely at any time as long as the concerned sim isn't currently in alien form or in dark form.