There are eight ways to get three in a row: three rows, three columns and two diagonals.
Look for a line that already has two X's with the third square still free.
The diagonals get overlooked most often - check them once more at the end.
Examples
Example
Double threat
With a double threat you do not win right away.
You place an X so that afterwards you could win on two different squares. O can only block one of them, and you win on the other one with your next move.
If O already threatens three in a row, you have to block there first: then the right move is the one that blocks and sets up the double threat at the same time.
In the example, after the move on 7, X threatens on 4 and on 5.
For parents
What does this exercise practice?
The winning move practices thinking ahead: play the next move in your head first, then decide.
Common pitfalls
Some children look for the move that blocks the opponent. What is asked for is the move that lets X win; X only has to block when O already threatens three in a row.
How to practice at home
Play a few rounds of noughts and crosses and say out loud before each move which line you are aiming at.