Making the "board" will keep the kids occupied for a while; and so will playing the game. Siblings of different ages can play this with each other quite happily.
Age: 5+ (with adult help)
You will need a large box lid (or, at a pinch, a large cereal box). Mark 5 circles on the lid of varying sizes, the first only just big enough for a marble to drop through, the others increasing by size. Paint the lid if you wish. Using a large felt-tip or marker pen, Number the largest hole 5; the next largest 10; the next, 20; the next, 50; and the smallest, 100.
Place the box on the floor and give each child an equal number of marbles. The object of the game is to see which child can count the most by dropping the marbles into the box through the holes.
Each player in turn stands over the box, holds his arm out straight, even with the shoulder, and drops the marbles one by one into the box. If one goes through the largest hole it counts 5, if through the smallest, 100, and so on, count being kept for each player. The one scoring the greatest number of points is the winner.
On the whole, the age of the children doesn't matter - as children grow older and taller, their skill level should increase too and the benefits of being shorter should even out!