If a toddler develops diarrhea and has no other symptoms it can be caused by several different things such as infection, bacteria, eating too much fiber, drinking too much milk etc.
The diarrhea could be an onset of the stomach flu as well and if the diarrhea lasts longer than a few days then you should take the toddler to the doctor to have them checked out.
Other things that can cause diarrhea in toddlers are things such as rotavirus, bacteria like salmonella and sometimes but rarely, parasites like giardia.
The toddler may also have some disorders of their GI tract which can lead to the toddler developing diarrhea.
I wouldn't worry about the toddlers diarrhea unless it lasts longer than a few days to a week but if you're worried it may be caused by something more serious then taking the child to the doctor or urgent care would be a good idea just to be safe.