Thank you for your prompt reply and answer.
My granddaughter is aged 9 and seems quite keen on bugs and wildlife at the moment.
You mention about one species that lives on the seashore and feeds on seaweed. As a child I lived in a small North Cornwall fishing village and on the seashore at low tide there were sand hoppers that lived under seaweed. Are these sand hoppers the species mentioned or are they a different species?
It's great to hear your granddaughter is keen on bugs and wildlife. Do you think you'd be interested in doing a pond survey with her? We've got a really nice pack that's been developed through our OPAL project (http://www.opalexplorenature.org/?q=watersurvey).If you would be interested, I'll send one to you if you let me have your contact details (my email is firstname.lastname@example.org).
I too grew up on the coast and have brilliant memories of chasing sand hoppers over the sand. They are very closely related to woodlice, both belonging to the Malacostraca, a class that also includes crabs and lobsters. For more info on sand hoppers you could have a look at http://www.arkive.org/sand-hopper/talitrus-saltator/
With best wishes.