Millet — Food safety facts
Safe to share · Dogs · full breakdown on the linked food page
Cooked plain millet is safe and gluten-free.
The full verdict, portion guidance and serving notes are collected on the Millet food facts page.
Short answer
No, you don't need to worry about the others. Plain cooked millet is safe for dogs, so the one who ate it will be fine. Just offer the others some fresh water and watch all of them for any signs of an upset stomach. If any dog seems off, give your regular vet a call.