There are few things in grammar that cause as much misery as the apostrophe. For some reason beyond my understanding, kids are introduced to the apostrophe, possessives, and contractions at the same time. The result: Complete confusion.
Now that you’ve introduced your kid to the apostrophe and possessives, it’s time to tackle the apostrophe and contractions.

What is a contraction?
A contraction is formed when two words are presented as one, with the apostrophe taking the place of the missing letter or letters.

How it works
cannot = can’t
— the apostrophe takes the place of no

How it works
would not = wouldn’t
— the apostrophe takes the place of the o
should not = shouldn’t
— the apostrophe takes the place of the o

How it works
will not = won’t
— the apostrophe takes the place of the o
do not = don’t
— the apostrophe takes the place of the o

How it works
it is = it’s
— the apostrophe takes the place of the i

Hint – Hint – Hint – Hint
Do not have your kid use the apostrophe for contractions until they grasp the concept of the apostrophe for the possessive. How? Have them write do not rather than don’t, should not rather than shouldn’t, etc. Once they get it, go for the apostrophe whenever it’s needed!