The grandchildren would be the parents, aunts and uncles, and first cousins once removed, of the great grandchildren.
Your grandchild and your brother's grandchild are second cousins.
Your grandchild and your niece are first cousins, once removed.
Does the stepchild and your child share a parent? If so, they are half-brothers, which would make the grandchild your son's nephew.
You are his grandchild.
Your first cousin's grandchild is your first cousin, twice removed.
You and your uncle's great grandchild are first cousin, twice removed.
The grandchild of your great grandmother's sister is your second cousin, once removed.
Great aunt or great uncle depending on your gender.
If the daughter is biologically related to you, it would be your great-grandchild. If your husband had his daughter before he married you (which would make her your step-daughter), it would be your step-great-grandchild.
You would be Second Cousins
You and your uncle's child are first cousins to each other. You and your uncle's grandchild are first cousins, once removed, to each other.
Your grandmother's sister is your great aunt. Her son's child is her grandchild. You and your great aunt's grandchild are second cousins.
You are great great aunt to your niece's grandchild. Your children and your niece's grandchild are first cousins, twice removed.