Rails - only find records where has_many associated records are present

I have a model with a has_many relation with another model like so:

class Parent < ActiveRecord::Base
  has_many :children
end

class Child < ActiveRecord::Base
  belongs_to :parent
end

Since there are some parents that may not have children, I'd like to do a query that returns only the parents that DO have children. How would I go about something like this?

Since I'm doing this on Rails 3, it would be helpful if this query were to not use the where.not. syntax.