"I won't. It- it would worry them too much, I fear." It worried him, even, and Aziraphale was suddenly annoyed at himself for being so frightened. "I'll keep it to myself, you have nothing to fear from me." Although I still disagree that they ought to be here.

He wouldn't tell Crowley about this. Crowley was too close to John, and while Aziraphale could swallow this decision, Crowley may not be able to. If anything happened to Adam - despite Adam's reassurances - then Aziraphale would know, and could prevent disaster. There was comfort in that thought. He didn't require Crowley to worry about it as well.