Your Wrong!
I think Krillin would be hard, 'cause they have to somehow remove his nose, and CG would be too noticeably, and a mask would look unrealistical.
As it goes for Gohan, they might not show him w/o Clothes, so there are more than that Mini-Bodybuilder, that can play him.
This is kinda on topic. How will they do Trunks' hair?
i think that gohan would be the hardest to translate on a big screen because to get a 5 year old kid to smash a body builder like nappa into a mountain would be kinda hard, but they would have to alter the age of gohan to have a child star fight.
Krillin, IMO.
The nose thing would look weird on screen but how else could you make Krillin without that physical feature? Its what makes him him ... kinda.
With a nose he would look like any ordinary shaolin monk.