The child usually needs nothing: boards, clocks and scoresheets are provided. Check one thing in advance - whether they need a pen for recording games - about notation.
At most junior tournaments parents are not allowed into the playing area, and that is not organizer strictness but necessity: twenty adults standing over the tables distract every child at once.
The general conduct rules are the same as for adult players - chess etiquette.
This is the one section about rules rather than tact. Prompting from the sidelines is a violation, and the player is the one who answers for it.
What can happen
The arbiter may award a loss to the child - the one who was prompted. So a parent trying to help takes a point away from their own child, and sometimes a place in the tournament.
Prompting is not only words. A nod, a sigh, a glance at the right piece, an «oh» at the moment of a blunder - children read all of it, and arbiters see it. The safest place to be is one from which the boards are not visible at all.
Separately: do not go through the position with your child during a break if the game is still adjourned or continuing. That is the same thing, spread over time.
Three sentences worth avoiding, because they achieve the opposite of what is intended:
Children cry at junior tournaments, and that is normal: losing at seven is genuine grief, not a tantrum. What matters is what happens next.
If it happens at every tournament, that is a signal that competitive play is premature rather than that chess is not for them - readiness for tournaments.
And ask whether there is a meal break: at events longer than three hours there usually is, but not always - how junior events are run.
And the most valuable thing you can do: take an interest in the games rather than the result. «Show me how you won?» does more for a child's relationship with chess than everything else on this page combined - what chess really gives children.
At most junior tournaments parents are not admitted to the playing area - twenty adults over the tables distract every child at once. Where you are admitted, keep your distance: do not stand behind your child, lean over the board or approach while they are thinking.
The arbiter may award a loss - to the child who was prompted. So the parent takes a point away from their own child. And prompting is not only words: a nod, a sigh, a glance at a piece or an «oh» at the moment of a blunder all count.
«It happens. What went on there?» - and then listen rather than explain. Avoid «how could you hang your queen», comparisons with other children and reminders of how much you have invested: all of it teaches them not to show you their games.
Take them calmly out of the hall, acknowledge the feeling and move on: «that's upsetting, I understand, let's get some water». Do not over-console, do not look for someone to blame and do not dismiss it as «only a game». Do not take them home unless they ask.
Rounds multiplied by twice the time control, plus breaks. Five rounds of 15 minutes is about three hours including the prize-giving, and junior events almost always overrun - allow slack.
Better not: to save half an hour the child loses the moment their name is called. If you genuinely have to go, tell the arbiter in advance so any prize or certificate can be handed over earlier.
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For parents - what age to start, what chess gives children, children online, choosing a coach.