There are a number of traits I would recommend using when fielding Pre-Heresy Emperor's Children army. All information is taken from 'Codex: Space Marines' Copyright Games Workshop Limited 2004.
First and foremost I would recommend 'Trust Your Battle-Brothers' meaning that for +3pts per model all Command, Veteran, Tactical, Assault and Devestator Squads may benefit from Counter-attack and True Grit. This could be seen to be representative, less of a desire to protect one's Battle-brothers, but to gain a fair share of the glory for oneself.
Secondly I would recommend taking 'Purity Above All', which allows any Veteran or Tactical Squad to upgrade its Veteran Sergeant to an Apothecary for + 25pts, including a Narthecium and Reductor. The reasons for this are twofold: firstly the Legion was nearly wiped out at its inception, so it could be said that the Apothecaries take to the field of battle to ensure the survival of as many Battle-brothers as possible, and also to observe the rigours of war on the Children Of The Emperor. Secondly, if the army is set around the time of Istvaan III, it could be argued that the Apothecarian sends its men into batttle to monitor the stability and effectiveness of the Legions newest path.
You could also choose to use 'Uphold The Honour Of The Emperor' whereby Command and Veteran Squads may forgo the benefits of Cover to receive a 6+ Invulnerable save. This is representative of the Emperor's Children's own self-belief in their mission to cleanse the galaxy.
For the Drawbacks there are again a few choices - depending on whether you decide to take one or two of the Traits suggested above.
You could take 'Flesh Over Steel' to represent the fact that fewer vehicles were available at the time in question. This Drawback restricts you to taking 0-1 Land Raider or Predator Destructor or Whirlwind or Vindicator, and Land Raider Crusaders and Predator Annihilators are not allowed. Also any unit mounted in a transport vehicle takes up a Fast Attack choice on the Force Organisation Chart.
For the Minor Drawback I would recommend using 'Have Pride In Your Colours' meaning that no models in the army may use the 'Scout' or 'Infiltrate' Universal Special Rules. This one is pretty self-explanatory, the Emperor's Children were infamous for their pride.
The other Minor Drawback I would recommend is 'We Stand Alone', disallowing Allies being taken with the army. Again this could represent the pride of the Emperor's Children and their refusal to fight alongside any they saw as lesser than themselves.
These are by no means hard and fast rules for fielding Pre-Heresy Emperor's Children armies, merely my ideas on the subject.
'Children Of The Emperor. Death To His Foes!'