Cool. I love the pictures and the idea of tattoos marking them as servants. Great stuff.
If they are Hill Dwarves you could certainly increase their Hit Dice (making it 1d10 and increasing hps to 5) and also add +1 Constitution. Might not mean much now, but I will be going with a Savage Worlds-style system regarding leveling them up.
We used 50% chance of extras under your control leveling when you do. BUT in Savage Worlds that only meant getting one thing/increase. For this game I would simply say increase or get 1 thing when they level.
Egs: +1 Hit Dice; +1 att (max +2); +1 to ability score (once-off at this stage); +1 to skill; add new skill; add feat.
(None of these should happen more than they do for an adventurer. And I might adjust the % depending on how much a part each extra plays).
Thinking further - you could distinguish between the retainers a little. Maybe one if personal servant (bathing, dressing, cleaning - he could even be developing into a body guard) and the other more like a major domo (organises things like trips, where to stay, deals with NPCs on your behalf). That way the second one should certainly be trained in Diplomacy.
In fact I am happy for you to choose up to 3 skills for each of them if you like.
In any case, this absolutely cool! I love the idea of Extras/NPCs with stats and ready to go in the players' hands. Something I loved from Savage Worlds. :)