Maybe they just heard a funny joke or saw something hilarious.
The cartoon boys might be laughing because they accomplished a difficult task and are celebrating. Maybe they saw an animal doing something comical. There are so many possible reasons!
It could be that she's playing with friends and having a great time, which made her laugh. Or perhaps she heard a joke that cracked her up.
It could be that they are having a great time playing together and something made them burst out laughing. Or perhaps they achieved a goal they were working towards and are celebrating with laughter.
Maybe the baby saw something funny or was being tickled.
Maybe it's having a great time playing or just saw something funny.
Maybe he just heard a funny joke and couldn't help but laugh.
The cartoon kids might be laughing because they got a surprise or were having a friendly competition and someone did something silly.
Maybe they just had a funny moment or saw something amusing that made them laugh.
Maybe she saw something funny or got a pleasant surprise.
Maybe the kid saw something funny or was playing a fun game.
Maybe it's because of a funny joke or a silly situation in the cartoon.