How do you smile the baby in petalburg city?

To make the baby smile in Petalburg City in Pokémon Emerald, you need to find the baby in the house to the right of the Pokémon Center. Speak to the baby, and then use a Pokémon that knows the move "Sweet Scent" or "Attract." This will prompt the baby to smile and react positively. Afterward, you can continue your adventures in the game!