Who has a claim as an heir? If there IS a Will, but a grandchild is NOT in the Will, do they still have a claim? Or is the Will "airtight?" Thanks.

If there's a will, only those named generally are those that inherit.  There are, however, ways that one might dispute a will, but you really need to speak with an attorney who practices in this area to find out if you have an adequate challenge to the will.