I might be mistaken, but I am 99% sure my toddler imitated her mom.
Her mom was greeting a neighbor while they were out front with that one word of simlish that sounds like "Val-i-day!"
Followed by a little chuckle. Immediately after, the toddler, who was heading for the house, said in her little mis-pronounced baby voice "bal-ee-day" in the same exact tone followed by a high pitched giggle that mimicked the little chuckle of the mom.
I havent heard it happen since so i dont know if it was a coincidence?! But if they imitate their parents' simlish, that is ADORABLE