Hi there
I found this list of foods that can turn stool red for toddlers:
Red Jell-O, red or grape Kool-Aid
Red candy, red licorice
Red cereals
Red frosting
Red food coloring
Beets
Cranberries
Fire Cheetos
Paprika
Red peppers
Rhubarb
Tomato juice or soup, tomato skin
If your 3yr old are any of these or had medication that lists stool discoloration in the side effects I wouldnt be too worried and just see if it goes away next time. If not it might be blood and that would be a reason to talk to your doctor .
Mee Yoong