The best time would be during the Declare Blockers step.
Your opponent had already announced their Blockers, so they can't rearrange the blockers now that your Creatures have double strike.
And, since it's before a Combat Damage step had begun, it would allow for your Creatures to assign Combat Damage in both Combat Damage steps.

https://tappedout.net/mtg-decks/1-goblin-2-goblins-6-goblins/