I think I see where you're coming from now. The rules for Summoning on page 221 of Players Handbook 2 say:
Attacks and Checks: If a summoning power allows the summoned creature to attack, you make an attack through the creature as specified in the power description.
So, by that logic, if your summoned creature attacks, YOU are attacking through it. If your fangs give you extra damage whenever YOU hit with a melee attack, and your creature makes a melee attack and hits, by the transitive property of attacking, YOU have just hit with a melee attack - ergo YOU get to deal an extra 2d6 damage. (Note that you still have to be in beast form in order for this to work.)
Now, by RAW (Rules As Written) I believe this works. However, if I were the DM I would say no, that doesn't work at my table. In my opinion, the flavor of the Serpent's Poison ability is that you, the druid, change into a giant serpent when you go into beast form, and that serpent has poisonous fangs that deal extra damage when they strike. The creature that you summon is not a giant serpent and therefore lacks the fangs and the poison.
But technically speaking, by a close reading of the Summoning rules, it does look like the creature's melee attacks would deal extra damage when they hit.