Firstly as you describe chances of ingesting rat poison bait in his case are less.
Even if he has consumed small amount just one pellet which is smaller than a pea so with such a small exposure, it would be VERY unlikely to have any affect on him whatsoever.
And, usually there would be some immediate symptoms like vomiting,diarrhea,bruising within few hours to 72 hours after ingestion.
So keep looking for any signs of bleeding or bruising or vomiting.
If the child is sleeping well and showing no signs of nausea , vomiting , avoiding to eat or drink milk , you should just follow a wait and watch approach.
If he develop any symptoms then visit ER.
I hope this helps.
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