Rails - Jun 2006 - validates_associated -- explicitly setting collection?

Hi, I have a class (say Parent) that has many of two other classes, say 
FirstChild and SecondChild. These could look like this:

class Parent < ActiveRecord::Base
  has_many :first_children, :dependent => true
  has_many :first_children, :dependent => true
  validates_associated :first_children
  validates_associated :second_children
end

class FirstChild < ActiveRecord::Base
  belongs_to :parent
  validates_format_of :birth_date, :with => /^\d{4}\/\d{2}\/\d{2}$/
end

class SecondChild < ActiveRecord::Base
  belongs_to :parent
  validates_format_of :birth_date, :with => /^\d{4}\/\d{2}\/\d{2}$/
end

The key bit here is the validation -- namely the validates_associated.
If I collect hashes of data from a form for the child classes, I should 
surely be able to assign them to the Parent class, and then have that 
fail validation if one of them fails?
Maybe like:

parent = Parent.find(params[:id])
first_children = FirstChild.update(
    params[:first_child].keys, params[:first_child].values)
second_children = SecondChild.update(
    params[:second_child].keys, params[:second_child].values)
parent.first_children = first_children
parent.second_children = second_children
# if I debug here, none of the errors in the local arrays first_children 
or
# second_children are in available in either parent.first_children or
# parent.second_children
if parent.update_attributes(params[:parent])
  # whatever...
else
  # should end up here if any errors in parent or child collections
end

(I''ve also tried a number of other ways to assign the collections, 
including ActiveRecord::Base.update_attribute(), but none of them work.)

Can anyone help me to accomplish what I need to do here?
If this can''t be made to work, them I don''t really see any
point to
validates_associated.
Any help *very* gratefully received by a *very* tired programmer!
Cheers,
   doug.

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