Only the plants belonging to the groups Gymnosperms and Angiosperms have seed dispensers (think of pine cones and apples). The seaweeds (kelps and rock weeds) have a very different means of reproduction that doesn’t involve the production of seeds.
What you have found most likely are the egg cases of a shark or skate, which are often called “mermaid’s purses”. This link describes egg cases in some detail and includes images to compare with what you have found. To help further your grandchildren’s interest in sharks and marine life, check out the Shark Research Institute, which is located in Princeton, NJ, and select “Kids” from the menu.