children() is supposed to take a function as argument and that function is supposed to return the list of children.

So basically:

children( function(el) {

  return [

    el.email = input(props({ type: 'email' })),

    el.pass = input(props({ type: 'pass' })),

    el.submit = button(text('Sign in'))

  ]

Also,

[

   el.email = input(props({ type: 'email' }))

   ,

   ...

is a neat idea. In one (expressive) line he is assigning the value to the array and also giving the child (in this case, email) a name and reference in its parent, so later he can just call el.email (in the onsubmit function).