If you have several children with Seesaw Learning Journals, you can switch between them in the panel on the left (this is also how the children switch between different classes in Seesaw, from Grade 1 up).
You can join each of your children's LJs using the link your teacher sends you, or you can copy the link and add it yourself in Seesaw in any browser, where it says +ADD JOURNAL
For more from Seesaw, see this link.