How do I change a vampire toddler to a human?

I'm not sure what you mean by lose emotions? I've used the "humanize sim" command in MCCC several times with no ill effect on the sim other than to, well, humanize them. I'm not sure if it works on a child or toddler, though, since their powers don't manifest until they hit teenhood. In that case you could use it as soon as they have their birthday, or have the drink ready for them at the time. Until that point they are exactly like any non-vampire sim except for the vampire symbols on the needs panel and in CAS.

I don't know where do you take the fact that vampires turned into humans by cheats (or MCCC) makes them not feel emotions? I think it only happens when vampire has the Dampened Emotions power, but babies, toddlers and children cannot have powers and weaknesses yet.
Vampire children live normal children until they become teens. Then you can cure them with the vampire cure. Children can also learn the Vampire Lore skill without needing to max out one of the child skills.

@simpatsyann @EgonVM
I tested with a toddler and cheated her needs very low but she wasn't angry or sad. She didn't have any moodlets she was just... fine. She doesn't get any moodlets. In MCCC it said at the description that sims lose all their emotions.

"Toreshinbo;c-16273459" wrote:
@simpatsyann @EgonVM
I tested with a toddler and cheated her needs very low but she wasn't angry or sad. She didn't have any moodlets she was just... fine. She doesn't get any moodlets. In MCCC it said at the description that sims lose all their emotions.

Ok. I understand. That's strange...
Well then, just pretend that the vampire child is a normal child and when they become a teen, cure them.
I'll have to test out this cheat below and see if it results in loss of emotions.

traits.remove_trait trait_OccultVampire

Here are the results:
I removed toddler's vampirism and waited until her needs got low. Her fun need got low and she got sad like any normal toddler.
Now I can clearly tell that this issue is caused by this mod (Or by other mod).

@EgonVM I went into the save again and have reset both toddlers. Then I restarted the game and they have their emotions now. The toddler had still low needs from last time and she was very pissed :lol:

For future vampire kids I'll just make them drink the cure when they're teens... Easier that way.