You left them a map of your treasure. How do you make sure that only one or two can't find it?
Make a circle, with the treasure at some point of the circumference (for example the southernmost). Mark 5 points on that circle. Give the direction of a point to each of your descendants. Now by geometry they HAVE to have 3 or more points to find where is the treasure (as 2 points can't really reconstruct a circle, you need at least 3!)