Select objects where all children have no grandchildren

I have the following three-level hierarchy of models:

class Parent < AR::Base
  has_many :children
end

class Child < AR::Base
  has_many :grandchildren
  belongs_to :parent

  attr_accessible :my_number
end

class Grandchild < AR::Base
  belongs_to :child
end

It is expected that all Parents will have multiple children, but each Child may or may not have any grandchildren.

I want to retrieve all Parent objects for which all the children have no grandchildren. How can I do this? I'd prefer a Rails-way, and I have squeel available; but I'll settle for raw SQL.

Bonus points if you can give me all Parent objects for which all the children have no grandchildren and all the children have my_number < 5.