For a first attempt, pairs scoring is the easiest: it removes the «why is my child playing an adult» question while keeping ordinary pairings.
Four devices, simplest first.
The time handicap is the most practical of the four: it is set on the clock and requires neither an explanation nor a negotiation at the board.
Aim for 10+5 or 15+10 and four or five rounds: children will not manage more, and adults will not get bored. An hour and a half to two hours of play plus a break and the prizes fits inside a family weekend day.
Build in a pause in the middle, with food, drinks and room to walk about. At a family event the break is not a technical necessity - it is part of the point: people came to spend time together.
Working out the timings - the schedule.
The best time is a weekend morning or early afternoon: by the evening the children are tired and the adults are looking at their watches.
Prizes work differently here than at a sporting event: the occasion matters more than the value. What lands well:
One more thing: decide in advance what happens if somebody in a family does not turn up. The simple rule is that a pair or team may play short, with points counted from the games actually played.
A time handicap is easiest: the adult gets less time, say 5 minutes against 15. Separate prize categories, a material handicap by agreement, and Chess960 - where opening knowledge gives no advantage - all work too.
For a first attempt, pairs scoring: an adult and a child play their own games and the points are added. It removes the unequal-strength question while keeping ordinary pairings. With a large entry, family teams work better.
Four or five at 10+5 or 15+10, with a compulsory break in the middle. An hour and a half to two hours of play plus the prize-giving is right for a weekend day with children.
A weekend morning or early afternoon. By the evening the children are tired and irritable and the adults are in a hurry to get home.
Announce the rule in advance: a pair or team may play short, with points counted from the games actually played. Otherwise you are deciding it on the day, which always looks unfair to somebody.
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Children - junior tournaments and preschoolers. Beginners - a beginners' event. Formats - at home, outdoors.