The common usage in wills and trusts is "to my then living descendants, per stirpes."
What this language means is that if you have two children and five grandchildren who survive you, then each of your children will receive a 1/2 share and the grandchildren will receive nothing. If, however, one of your children predeceases you and is the parent of three of the grandchildren, then the surviving child will receive a 1/2 share and each grandchild will receive a 1/6 share (in other words, the deceased child's 1/2 share will be divided equally among the three children who have survived the deceased child: 1/2 divided by 3 = 1/6 each).
Contrast this to "per capita," or by head count: "to my then living descendants, per capita." In the above example with two children and five grandchildren who survive you, the children and grandchildren will each receive a 1/7 share, while in the example with one child predeceasing you, the surviving child and grandchildren will each receive a 1/6 share.