Why head-swap comedy needs a straight face
The format only works if exactly one thing is wrong. Keeping your head photographically unchanged — same features, same hair, same lighting logic — is what makes the baby body read as an intrusion rather than a cartoon. Stylise both halves and it stops being funny, because there is nothing left to be surprised by.
Source photos that map well
Shoot or pick straight-on. The head is being fitted to a proportion far from an adult's, and a three-quarter or upward angle gives the model a mapping problem on top of a scaling one, which usually shows as a stretched jaw. Hair that is clearly separated from the background also helps, because the silhouette has to survive being scaled against a much rounder body.