North West, Penelope Disick: Casual Mermaid Attire Required For Our Birthdays

North West's 3rd birthday train rolled on Saturday in fancy Bel Air ... and she definitely made a splash with cousin Penelope Disick who's also celebrating a birthday soon.

Kim and Kanye threw the shindig for their daughter, continuing the Disney-themed celebration ... mimicking "The Little Mermaid." Penelope will celebrate her bday early next month and we're told the bash was a combo for both girls.

The fam hit up Disneyland Thursday. Walt would be pleased.