Kim Kardashian: Motherhood's a Beach with a Great Ass!

Kim Kardashian slipped into a tiny bikini and hit the beach in Mexico ready to roll around in the sand ... with her kids.

Yeah, North and Saint are on vacay with mom. The whole fam went out Thursday for some fun in the sun -- and the kids are so freakin' adorable they almost upstaged Kim's famed assets.

Almost.