I love that you are here asking. I'm with the others, the kids are not going to be the ones who will care for it. Sadly the parents will probably resent and be pretty upset with your wife if she does not have their permission.
I'm the type that loves all animals, I have almost anything my kids or myself would ever want. Minus the goats and cows because we live in town they they frown on that. I'm still tempted to get a mini house cow. I read they can be house train too But here's the kicker, if my inlaws showed up with a cat for my kids I'd kill them. Anything that comes in this house I'm the one expected to care for it. Sure the kids help out, but it is all on me the parent. Please try to convince your wife not to do it unless she gets the go ahead from the parents. As Lemonslice mentions the Sphynx are very high maintenance, my son is waiting for but not until he buys his house because his apartment is super cold.