My reasoning so far
OOC- Just in case we've forgotten, the penalties for breaking a Mortal Severity oath are as follows.
I have not forgotten the penalties that accompanied the benefits of breaking the oath - if anyone has, it's Archonus, who seems to take his oath(s) relatively lightly, despite their eternal (and severe) nature. He dedicated himself to a cause, declared it to be his life's purpose, etc. And now that it has become difficult to reconcile his responsibilities and desires, he has chosen the one to which he has no oath, only desire.
The basic violation penalty is a -5 to attack and damage rolls and to either skill checks or to saves (GM's choice) as well as suffering 3d6 points of damage that cannot be healed in any way until he resumes complying with the oath's terms. The penalty lasts for a week.
He has a significant chance to have broken this oath, and the fact that the penalty is severe should have either deterred you all from taking the oath in the first place, or at least urged caution in treading 'round the things that could break it.
Lose the morale bonus and the ability to locate the liege, but messages still reach him and his liege can still locate him, although he can no longer locate his liege. Suffer a morale penalty to all Will saves equal to the severity rating.
Putting the child above the circle probably does not break his oath to Jaine - she considers the survival of the child (and her line) of paramount interest (which is why he is being told to take care of the daughter), so she'd probably view this as fulfilling her oath in some way.
HOWEVER - as long as the child's safety were not severely or obviously compromised by Archonus's actions, she'd probably consider a balance of priorities (or considering the cause of the war paramount) a fulfillment as well. In either case, he's doing something for Jaine.
The penalties only affect the person guilty of violating the oath's terms, who is branded with an indelible sigil on his forehead, marking himas an oath-breaker and a traitor. He suffers a -2 morale penalty to attack rolls and all Charisma-based checks, with all NPC attitudes starting at one stage worse than they normally would. Every member of the oath's circle is aware of the character's treason the moment he commits it and gains a +2 morale bonus on all dice rolls made against him.
Once more, the severity of the penalty is meant to be a deterrent to the characters that take these powerful oaths from breaking them, not to the Universe for enforcing them. He may not have joined the enemy, but that's not the only condition through which the oath could be broken - only one of the most severe. Unless I can be convinced that subordinating the cause, the war, and the Circle to the child is actually a fulfillment of the letter and spirit of the Oath of Alliance (or is it Friendship?) that created the Circle, he has violated its terms, and cast aside his sacred honor for the love of an unborn child.
This is not necessarily an EVIL act, but it still breaks the oath (so far, that's my feeling, anyway).