Will my Pelakka Wurm still deal trample damage vs Deathtouch?
All combat damage happens at the same time.

Your son assigns the damage with the Eldrazi. You assign the damage with Pelakka Wurm. If Wurm's power is higher then Eldrazi's toughness you'll be able to assign the surplus to your son.

Then all damage is dealt at the same time. Your wurm dies, as Eldrazi had deathtouch. But the Wurm also does it's damage to the Eldrazi and, possibly, your son.

If Eldrazi had First Stike - that would be a different story.