"It appeared to be a magical contract."
Yes, that's what it looked like to me too.
Adalind got her powers back...so Stefania performed her side of the contract. And when one side performs on their side of a contract, that usually means that the contract is in force. Adalind is in breach of the contract because she didn't fulfill her side of the deal.
I'm thinking that Stefania probably sold her position in the contract to the Royals. Or maybe she just gave it to them, signed it over somehow, in exchange for her life.
Victor referred to baby Diana as "our child" a couple of times.
It's some sort of 'magical' contract...so it probably has some sort of binding power, much more so than a contract that you could try to enforce through the courts. It is illegal to sell a baby in most countries, anyway.
Adalind was right when she told Sean Renard that the Royals "won't need her anymore" once they get Diana. Renard doesn't know about the 'contract', does he?
Renard knew that Adalind was trying to sell the baby...but I don't remember him knowing about the contract.