Hello,
That seems like an unusual reaction.... Is it just in hearing, not seeing other kids? Only babies? Could you give me another example? Did he accidentally see part of Chucky the movie!
I know its hard with a 2yr old, but have you talked about it later with him or asked his caregivers if they can think of an situation that might of prompted this.
My 2yr old was very fearful of tall skinny men and wouldn't even go into a room with one. I never figured out what was the exact issue or incident, just tried to work with him to move past it.....
I don't envy being on the plane with a scared little guy like that!

it seems to only be when he can't see the other child. seems to be babies up to 3 or 4 year olds. older kids he's fine with maybe because they don't tend to make unexpected noise like the younger ones?

on the plane i kept pointing to the baby and saying, "there is a baby, can you see her? she makes noise like in your book, like the baby at the park. its scary when you don't see the baby." he has this book called "what does baby say" that he likes so i tried to reference that. also he's really fond of a baby that we see often at the park and i mentioned her.

happened again today at the supermarket. he was riding on my back and a baby was behind us and made some noise. ds got really upset again and i just repeated, "its a baby, you were scared because you didn't see the baby.

on a similar note, he's always gotten startled pretty easily. if the cat makes a noise or dh walks in the room unexpectedly he runs to me. i do the same validating and telling him who it is making the noise. but its nothing compared to the reaction he has when another kid makes the noise.