Hey Crin!
I tried this again in a new save with no mods or CC. I got the fear triggered within less than a minute of gameplay where the non-Jealous parent interacted with the infant for a few social interactions (I'd say 3-4).
For control, when I aged up the infant, I had the couple interact after every 3 interactions the non-Jealous parent had with the toddler. Additionally, I locked the family in a room with no objects other than the ones used to take care of the child/survive so the Jealous parent could always watch. With the toddler, the reaction and fear also triggered, however, it took much longer (around half a Sim day).
Interestingly, when the toddler aged up into a child, no reaction or fear was triggered, despite for a whole day the non-Jealous Sim played dolls with the child.
I get that for Jealous Sims, the Jealous Pangs moodlet and reaction are "not always reasonable" (as per moodlet description) but it's still weird that Sims fear their spouse cheating on them when they hang out with their kids.
