Kids sign in with a short code instead of an email and password. Find it on your child's page, then on their tablet or phone tap “I have a code” (on the web, choose “I'm a kid” on the sign-in page) and type it in.
Making a new code turns the old one off straight away, so a lost tablet can't keep using it.
What kids can do
You decide per child: capture pages, write their own stories, make pictures, pick picture styles, delete things. Defaults are careful; open them up as your child grows into them.