An English Teacher Rants About Proper Word Usage: Rant #1 "Toddler"

Yes, I am silently correcting your grammar, syntax, punctuation, vocabulary use, etc. all the time.  I cannot help it any more than a dentist can help noticing the state of your teeth or a fashionista can help noticing whether or not you have run-down heels.  And the internet is FULL of mistakes which make me want to slap people.  However, since slapping my laptop screen will do no good, I will rant instead.
Rant #1 concerns the misuse of the term "toddler."

A toddler is a child who is past the stage of infant.  It is a child who is learning to walk, and hence, it toddles, or walks unsteadily.

From a medical dictionary:
toddler

Etymology: ME, toteren, to walk unsteadily

a child between 12 and 36 months of age. 

From a Random House dictionary:

toddler  (ˈtɒdlə)
— n
1.	a young child, usually one between the ages of one and two and a half

Thus, unless the kid can't walk properly after the age of 2 1/2 or 3, it is not correct to call that kid a "toddler."  If it doesn't toddle, it's not a toddler.
A child too old to be a toddler but too young for school is often called a pre-schooler.  This general designation indicated children from age 3 to those 5-year-olds not yet in kindergarten.  A child of 3, 4, or 5 is NOT A TODDLER.

This is wrong.

So is this.

Learn the difference and stop confusing your readers.  There is no such thing as a 4-year-old toddler.