Why a small bowl is no concern for a 5 kg dog
Your Shih Tzu's size and the amount eaten make this a non-event. Dogs digest grains like millet easily when cooked, and a few mouthfuls won't overload their system. The only possible reaction would be mild stomach upset if he isn't used to fiber, but even that's unlikely with such a small quantity.
Watch his next two meals for any hesitation to eat or slight changes in stool consistency. These would be temporary and resolve without intervention. If he acts perfectly normal (begging for treats, playing as usual), you can stop monitoring after his next poop.
When bland food helps instead of harms
Cooked millet is so neutral that vets sometimes recommend similar grains for dogs with sensitive stomachs. Unlike rich or fatty human foods that cause pancreatitis, plain grains provide safe calories. Your dog might actually benefit from the extra fiber if his regular diet is low in it.
That said, any vomiting or diarrhea would point to an unrelated issue—perhaps he ate something else you didn't witness. True millet intolerance is extremely rare in dogs. Just keep fresh water available and skip one meal if his belly seems off.
The twelve-hour window for small dogs
With a petite 5 kg body, anything he ate will pass through quickly. Most digestion happens in the first six hours post-meal for small breeds. You'd notice gassiness or discomfort by then if it were going to occur.
By the twelve-hour mark, the millet has either been absorbed or moved to the colon. Loose stools at this stage would suggest his gut flora reacted mildly, but again, this is uncommon with plain cooked grains. No news is good news after half a day.
Why you don't need to call the vet yet
Emergency vets see dogs for chocolate, grapes, and xylitol—not bland carbohydrates. Unless your Shih Tzu starts drooling excessively, collapsing, or showing neurological signs (which wouldn't be from millet), this doesn't warrant a clinic visit.
Save the vet call for if he stops drinking water or can't keep anything down for over 12 hours. Those would indicate an obstruction or illness unrelated to the millet. For now, just enjoy watching him nap off his snack.
The full verdict, portion guidance and serving notes are collected on the Millet food facts page.