There are so many things that can cause a toddler to have diarrhoea including:
A milk allergy or being lactose intolerant may be the cause of diarrhoea in a toddler. Diarrhoea can also be caused by an infection called Giardiasis. If the diarrhoea lasts longer than what you consider normal, you should likely see your Paediatrician. They will do stool cultures to check for parasites and bacterial infections. Also, if your child is very fussy, has greasy stools that are very foul smelling, or if he is not gaining weight well, then your paediatrician might be more aggressive in looking for a medical cause for your child's loose stools.
Another extremely common cause of diarrhoea at this age is simply called toddler's diarrhoea, which is caused by drinking too much fruit juice. This type of diarrhoea usually begins between the ages of 6 and 30 months and goes away by the time children are about 4 years old. A child with Toddlers Diarrhoea can have 2 to 6 watery stools each day, but have no other signs of illness and they gain weight normally.
There are several things you can do to help your toddler get over diarrhoea before using over the counter medications:
Limit the amount of fruit juice or take it out of your child’s diet completely. Juices with high levels of fructose or sorbital, like apple juice and pear juice, should be eliminated and replaced with better juices such as white grape juice.
Adding more fat to a child’s diet is known to help cure diarrhoea, although this is something you should discuss with your paediatrician to make sure you are not giving your child too many high fat foods, which could cause other problems.
Although fibre is known to help with constipation, it also helps many different kinds of gastrointestinal disorders. If you increase the amount of fibre in your child’s diet, do it gradually.