Hi and WELCOME to YT

! I would go back to her previous food to see if she goes back to feeling/acting normal. If the bobble head stuff is still there, you absolutely MUST see a vet asap as it could be liver shunt symptoms or something neuro or neck related. After vomiting, she also could've been hypoglycemic - so if that happens again, I'd give her Nutrical or Karo Syrup right away.
Lastly, be VERY careful with having a 3 yr old around such a tiny dog - a young child can easily and quickly hurt a tiny dog, without meaning to. Never let the dog alone w/ your child.