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The previous expert is correct, in that the amount of onion that was ingested should not cause a problem for your cats. However, the fact that the ingestion of onions in cats can cause anemia, it would be a good idea for you to have the cats seen in person by their vet, tomorrow, first thing, and ask if they should have a routine blood test, just to be sure. However, it might be too early to see any change in red blood cells at this time.
Inducing vomiting is not necessary, but you might expect to see an upset stomach, diarrhea, vomiting, since this is not something they have ever eaten before.
I would monitor them carefully tonight and if you see any unusual symptoms, bring them to the nearest ER vet center to be checked out. If they seem fine and are eating, drinking, eliminating, playing, etc., bring them to the vet tomorrow, just to be on the safe side.
I hope all will be well with both your cats!
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