habtm problem

I think we need to see one your kid/sport model and then the new/
create method in the controller...

You should not have a id column btw in the kids_sports table.. .

This should be your up task:

  def self.up
    create_table :kids_sports, :id => false do |t|
      t.integer :kid_id
      t.integer :sport_id
    end
    add_index :kids_sports, [:kid_id]
    add_index :kids_sports, [:sport_id]
  end