Once you drop the first kid on the crab they'll start 'distracting' it. There's plenty of time to grab a second kid, and then an adult to do the actual catching. No need to pause the game.

the crab appears before dropping the first kid on it. I hold the kid by the beach above the pier and wait for the crab to jump up. As soon as it does, I drop the kid, quickly grab another kid and drop him/her in the same place. Then you get an adult. You have to repeat this five times