You need to have a straight talk to your ILs. Explain what you want and then ask if or how they can help. Can they come to yours? Can they have the kids at theirs? Tell them they are the only option that you really want, but that you would have to consider a Nanny otherwise. Offer to pay the flights (it'd be less than a nanny) and maybe see if a friend can have one of the kids just for a night or two to help them out.
Have you booked your trip yet? Maybe it needs to be done as soon as you get the thumbs up from the ILs - then they can't back out etc (I have a history of someone backing out on me - drives me silly).

Nanny would be ok, but you would need to start the intro process asap.