Gosh I didn't know larkspur was deadly! We inherited a lovely deep blue form that self-sowed every year in our garden in Colchester, with labrador and toddlers running around!! Anyway, we (except of course dog long dead at a ripe old age not from larkspur poisoning) are alive and well to this day.
I did't pinch out they branch naturally. They might need a small stake to stop them falling over, and deadheading (or picking) keeps them going well. The petals also dry well for pot-pourri, no scent but lovely colour.
I love larkspur. Haven't managed to establish it self-sowing here, perhaps drier lighter soil suits it better?