My cousin came over who is 6 years old, and I went to get him something and when I came back he left bread on the floor, a huge bread piece on my desk. He told me he fed a tiny crumb to my betta, and of course moon eats everything, and he ate it....
Watch closely for bloating or other signs of distress.
I would also quarantine in a small container with epsom salt at 2 tsp/gal to help any bloat that does occur as well as to help flush out the bread if he didn't spit it out. While using the epsom salt, do 100% daily changes and redo the salt afterwards.
Small bloating, his belly sticks out a bit but he is acting normal, I have no salt or really much anything at the moment because I was getting a new tank for him... I only have Nutrafin Waste control, Nutrafrin Cycle, Nutrafin Aqua plus
He should be fine... just watch him like Pataflafla suggested.
Lots of fish food contain flour so it's not something that will poison him.
Skip today's feeding and if his belly is back to normal tomorrow, then start feeding again.
My 3 year old niece apparently has thrown bacon and bananas into her betta Yeyah's bowl and he is just fine. They knew she did it because she said "I know I'm not supposed to give Yeyah bacon" and low and behold .. there was bacon floating in there . . ..
(That's not supposed to be advice, ha ha. Yeyah is not usually within reach of the rugrats)