The ER doc gave my toddler a prescription of prednisolone. Seems like a big dose; any chance of overdosing on it?

If unsure... You should double check with the prescribing doctor, your child's pediatrician, or even the pharmacist who dispensed the prednisolone. A standard dose is 1-2mg per kilogram per day, but if it were me, i'd double check with your doctor or pharmacist just in case.