So this low-key makes no sense but:
Bruce has a metagene. It is that any cape he wears has a pocket dimension. The robins have an uncontrollable urge to hide in it.
It doesn't even need to be a legitimate cape. He was playing superheroes with a besheet when he was seven, and he pulled a medieval battle axe out of it. The Wayne's have never owned a medieval battle axe. Alfred, Thomas and Martha were extremely confused and concerned about it's origins.
Yes, I know Bruce isn't a meta. But it'd be really funny if he was. Especially with such a specific, weird power. That's how he fits all the stuff in his cape.
Like, snacks, and weapons, and Tim swears that one time he saw him just make a whole motorcycle appear from it.
Dick was cold on patrol once, and Bruce opened his cape, and just thought nothing of the fact that Dick just disappeared into it, dismissed it as a result of a really heavy warm cape.
Dick found himself in a warm, cozy, dark place, and immediately decided to go there as much as possible. He then told Jason, who didn't believe him at first but then found himself in the pocket dimension and was like "fuck yeah this is awesome".
He didn't tell Tim, and Tim didn't feel as if he was allowed to ask Batman to hide under his cape even when he was cold, so it took several years of vigilante-ing before Tim figured it out.
Damian didn't really have any chances to hide in Bruce's cape before he was lost in time, the cape didn't work the same for dick, and by the time Bruce was back, Damian believed he was too old for such childish things as seeking shelter in his father's cape.
The robins just think that Bruce got his cape enchanted somehow, and just didn't care to tell them in typical Bruce fashion. Bruce has no idea about this and is just happy that his kids like him enough to feel comfortable with him during patrol.