use MooseX::Declare

class MyCollection {
  has 'items' => (
    is => 'rw',
    isa => 'HashRef[Object]',
    traits => ['Hash'],
    defaults => sub { {} }
  )

    some..methods
}

use MooseX::Declare;

class Child {
  has 'name' => (
    is => 'rw',
    isa => 'Str'  
  );

  some..methods 
}

use MooseX::Declare;
use MyCollection

class Parent {

  has 'children => (
    is => 'rw',
    isa => 'MyCollection'
  )

  some..methods
}

##</code><code>##

subtype 'MyChildrenCollection' 
=> as 'Collection', 
where => { $_->isa('Child') }

##</code><code>##

has 'children' => (is => 'rw', 'isa' => 'MyChildrenCollection'