How is my daughter related to my father's (her grandfather) cousin's son's son?

3rd or 4th cousins.

Grandfather and his cousin are on the same level (1st cousin)
His child (you) and cousin's son are on the same level (2nd cousin)
His grandchild (your child) and cousin's grandchild are on the same level (3rd cousin)

So 3rd cousin

No 4th cousin

Grandfathers cousin would be mothers 1st, her daughers 2nd.
Then the son would be 3rd and the son/son would be 4th.

No 4th cousin

Grandfathers cousin would be mothers 1st, her daughers 2nd.
then the son would be 3rd and the son/son would be 4th.
Grandfather's cousin would be his own first.

Why would it be mother's first?