New to all this?
Start with the whole approach behind Confident Beginners.
See Confident BeginnersA foul is something one player does to another.
Explaining fouls and free kicks to a child comes down to two things they can hold in their heads. A foul is a kick, trip, push, hold or dangerous swing, and a free kick is the restart the other team gets, either direct or indirect. The wording is checked against Law 12 of the Laws of the Game, and the cards and the small print can wait until they ask.
For a long time I answered "why was that a free kick?" with "because the referee said so", which is true and explains nothing to a child.

Five words a child can hold
Five words cover most of what a young child will see: kick, trip, push, hold and dangerous play. They match the Law's own wording of kicks or attempts to kick, trips or attempts to trip, pushes, holds an opponent and plays in a dangerous manner.
The referee decides whether a challenge was careless, reckless or too forceful, so touching someone is not automatically a foul. That is the part worth teaching when you are teaching a child what counts as a foul: play "foul or fair?" with the situations below.
| What happens | Foul or fair? | Why |
|---|---|---|
| Kicks at the other player's leg, not the ball. | Foul | Kicking an opponent is a direct free kick offence. |
| Puts a foot out and the other player falls. | Foul | That is a trip. |
| Shoves the other player in the back. | Foul | That is a push. |
| Grabs the other player's shirt to stop them. | Foul | Holding an opponent is an offence. |
| Both go for the ball, one wins it cleanly and the other falls over. | Usually fair | Winning the ball fairly is not a foul. The referee judges. |
| Two players run shoulder to shoulder for the ball. | Usually fair | A charge only counts when it is careless, reckless or forceful. |
| A boot is swung high where a head is close. | Foul, dangerous play | Law 12 calls it playing in a dangerous manner, even without contact. |
| Stands still in a player's way without touching. | Usually fair | Standing in the way is not a foul. Moving into a player's way, or blocking them when the ball is out of reach, is an indirect free kick. |
One sentence, then a demo
Law 13 on free kicks sets out the rule, and the referee raises an arm above the head to show an indirect kick. Give them one sentence to say back: "A direct free kick can go straight in the goal, and an indirect one has to touch someone else first."
Kicks, trips, pushes and holds are direct. Dangerous play and blocking without contact are indirect. If a direct kick goes straight in, it is a goal, and if an indirect kick goes straight in, it is a goal kick, not a goal. When a defender commits a direct free kick foul inside their own penalty area, the restart is a penalty kick.
Try it with nobody touched. Any kick that goes wrong, just collect the ball and go again.
Goal. Set two shoes or cones as posts at one end of the lawn.
Cushion opponent. Put a cushion on the grass as the player who was fouled. Nobody is touched.
Direct kick. Place the ball on the grass and let them shoot straight at goal. It counts.
Indirect kick. Raise your arm above your head. They must pass to you or tap it to a second player before it goes in.
"Shall we do one direct and one indirect? Arm up means somebody else has to touch it first."
For rules that only apply in your own garden, see the house rules for keeping football fun.

Where the wording comes from
Law 12 awards a direct free kick for charging, jumping at, kicking, pushing, striking, tackling or tripping "in a manner considered by the referee to be careless, reckless or using excessive force", and for holding an opponent. A defender's foul of this kind inside their own penalty area is a penalty kick. Read Law 12 of the Laws of the Game. An indirect free kick is for playing in a dangerous manner or impeding an opponent without contact.
Law 13 says a direct kick can score straight away, an indirect kick needs a second touch, the arm is raised above the head and the ball must be still. Read Law 13 on free kicks. The FA's small-sided page lists the same direct offences in its own words for young players' games. Read the FA's small-sided version of Law 12.
The Laws are written for referees and the eleven-a-side game, the referee's judgement decides what is careless, and a garden game is not governed by them. Small-sided formats can sort free kicks differently, so check with your club or league. Parents often squash the direct and indirect rule into shorthand, so the Law's wording is the safer guide. Read a parent forum thread on explaining football rules simply.
When a free kick feels unfair
I used to defend the referee ("that was definitely a foul") the moment a child looked upset, which only made it about who was right. Name the feeling first, then name the one word: was it a kick, a trip, a push, a hold or a dangerous swing?
Be honest that referees decide in a moment and that children, and grown-ups, sometimes disagree with them. Then restart quickly.
"Shall we work out which one it was? Kick, trip, push, hold or dangerous?"
When they are upset: "That felt unfair, I know. The referee decides, so let's take the free kick and get on with it."
If it was your child who fouled, try "Shall we say sorry and carry on?" rather than a lecture. If the upset is about losing rather than one decision, the page on a child upset at losing football goes further.
Quick answers
Something a player does to another player: a kick, a trip, a push, a hold or dangerous play, when the referee judges it careless, reckless or too forceful.
A direct free kick can score straight into the goal. An indirect one has to touch another player first, and the referee raises an arm above the head to show it.
Because the referee saw one of the foul kinds above. It is the referee's judgement in the moment, which is why players sometimes disagree.
Use the five words, one cushion opponent and the garden demo, and play "foul or fair?" with everyday situations. Never act out a foul on the child.
If a defender commits a direct free kick offence inside their own penalty area, the restart is a penalty kick instead.
Keep going
Three places to go next, depending on what came up. Once fouls make sense, yellow and red cards explained to a child takes the next step. And how corners and goal kicks work is the next thing to explain.
Start with the whole approach behind Confident Beginners.
See Confident BeginnersOne sentence and a two-minute garden demo for the other rule children ask about.
See the offside pageWhat to say when the disappointment is bigger than one decision.
See the losing pageTonight's five minutes
Two shoes, a cushion, a soft ball and one raised arm.