If there are only a few kids in the draw (e.g. 4 or 5 kids) round robin would be good, anything larger than that use single elimination draw with first match consolation so they are guaranteed at least 2 matches. 8-game pro sets work well with young ones. No more than 2 matches per day per kid.
Another alternative would be to set up a team competition format. When I used to teach kids we used to to both. That way the kids who didn't do so well in the individual tournament weren't left out and it was fun for everyone involved because they can cheer their teammates on and kids of all abilities get to work together o nthe same team.