Balanced Encounter Poison Attack

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I am making a drake pet for my 7 year old son, who is playing a dragonborn Avenging Paladin. I'm giving him some lattitude on what it does and trying to balance it.

Base Stats
STR 16 CON 16 DEX 14 INT 3 (animal) WIS 14 CHA 14
At will attack Bite (D10) + STR (proficiency 3 as 2h sword)
Racial attribute bonuses +2 STR CON (included)
Racial skill bonuses +2 Endurance, Perception
Skills - Acrobatics, Athletics, Endurance, Perception
Natural Scale armor (it's a drake)
Equipment advances at L6, 11, 16, 21, 26, 30
+1/advance vicious weapon
+1/advance armor bonus (and add scale masterwork jumps at 21 and 30)
+1 item bonus to skills with armor advances
Racial Abilities
As Dragonborn, swap Trance (Eladrin) for Breath
Great Leap (as Rogue Utility 2)
Striker bonus +3 damage per tier if within 3 squares of master
Matching rolls - pet may use master's roll for any physical skill if they are together (so if toon jumps over ledge, pet will not drag down story, etc.)

What I want is a good poison bite as an encounter power - I plan to have only 1 Encounter Power and 1 Daily Power that is useable up to 4 times (Paragon + 3) eventually

Daily Power
Big Bite STR vs AC martial, reliable
Hit [3w] + STR, target is knocked prone if size large or smaller
Miss 1/2 damage
Change damage to 4w at 11, 5w at 21, and 6w at 30

Encounter Power
Poison Bite martial, interrupt, STR vs Fortitude
Trigger - successful hit
Hit:Target takes 5 poison immediately + 5 ongoing (save ends)
Miss: Target takes 5 poison immediately and 0 ongoing
Is this balanced for the heroic tier? What should the values be at each tier?