What does "divided among survivors of children or their issue, per stirpes". Specifically, 'their issue, per stirpes' mean ?

Sort of. Two points to make here. First, "issue" is a legal term that simply means lineal descendants. Second, ”per stirpes” is latin that just means means taking “by representation” or by branch of the family. In the will context, this means that if the beneficiaries share in a distribution “per stirpes,” the living member of the class of beneficiaries closest in relationship to the decedent will receive an equal share. If the member of the class of beneficiaries closest in relationship to the decedent has died, and is survived by any lineal descendants ("issue"), then that deceased beneficiary’s descendants will take “by representation” what their deceased parent would have taken. This makes the most sense when it is compared with "per capita," which means taking “by head count." "Per capita" means that all of the living members of the identified group will receive an equal share. If a member of the identified group has died, then a share won’t be created for that member, and all shares of the other members will be increased.

Mr. Fisk gives excellent advice. It is all blood descendants. One cannot stand in for an heir and take that share as a designee as there is no legal right. If a child 'designates' someone else to get his or her inheritance, it is a gift by them AFTER they receive the assets.

Yet another way to explain per stirpes is that "grandchildren" take less than "children." Although per stirpes does not apply ONLY to children, it applies to issue of named beneficiaries. For example, If someone leaves their estate to their 2 children, but 1 of the children dies before the testator (will maker) leaving children (grandchildren to the testator) those grandchildren take their parent's share. In this example those 2 grandchildren would each get 25% and the 1 child would get 50%.