To my understanding they are second cousins once removed. The degree of relationship between a pair of cousins is the number-minus-one of generations to the common ancestor for the person of the older generation (minus one as the first generation are siblings), 'removed' refers to the number of subsequent generations to reach the younger relative.

e.g. g-g-g grandchildren are five generations from a common ancestor so are fourth cousins

a g-g-g-grandchild and a g-g-g-g-g-grandchild are fourth cousins twice removed