My 2 1/2 year old has always liked them. Even on his 1st 4th and he was only 6 1/2 months old, he loved watching them go off. Last night, we watched outside for a bit, and when it was time to go inside for bed, he was watching them from his bedroom window for a little bit.
Our 4 month old, he seemed indifferent. I think he saw them, but didn't really react one way or another... so I guess no tears is a positive reaction.
Both fell asleep pretty easily considering it sounded like a war zone in our neighborhood.