u/Apprehensive_Bus1273

if Varys did try to swap Rhaenys here‘s how it would go:

Varys buys a random kid and gives her to Elia.

kid runs around the Red Keep yelling “I want my mommy! I hate this black cat! I don‘t know you, you’re not my mommy!”

random person in the red keep: “Rhaenys is sure acting weird.”

Yes, that’s why it’s hard to swap a toddler, because toddlers can walk, talk and have personalties.