They certainly can be. How much did Sara get into, and how recently? How is she acting now?
I haven't heard back from you in a couple hours, so I wanted to get you some more information.
Buckeyes can cause several symptoms, from gastroenteritis (vomiting, diarrhea) to neurologic signs such as coma, depression, excitement, etc. Decontamination within the first four hours or so after ingestion can limit toxicity dramatically.
Here is a link to a bit more info: http://www.aspca.org/pet-care/poison-control/plants/buckeye.html
Please let me know if you have any other questions or need any further information.