I somehow feel that Celestia knows very well what "toys" Twilight used to "play" with while she was her student. Twilight was always very close to Celestia and, especially in the times when she was in Celestia's School for Gifted Unicorns and slept there instead of being at home with her parents, I'm sure Twilight talked to Celestia a few times about things she'd normally talk to her mom about. And considering Equestria's society and culture, sex toys aren't such a taboo topic like in human society, so I'm sure the topic of Twilight's "toys" came up at some point.