The common usage in wills and trusts is "to my then living descendants, per capita." If you have two children and five grandchildren who survive you, then each will receive a 1/7 share. Or, if you have two children and five children and one child predeceases you and all others survive you, then the surviving child and grandchildren will each receive a 1/6 share.
Contrast this to "per stirpes," or by representation: "to my then living descendants, per stirpes." In the above example with two children and five grandchildren who survive you, if they take per stirpes then the children will each receive a 1/2 share and the grandchildren will receive nothing. If, however, one of the children predeceases you and is the parent of three of the grandchildren, then the surviving child will receive a 1/2 share and each child of the deceased child will receive a 1/6 share (in other words, the children of the deceased child will receive the deceased child's 1/2 share divided equally among the three of them: 1/2 divided by 3 = 1/6 each).