So I am looking at a property, only issue is the child of an existing tenant has high lead levels in his/her system.  The city came in and said the current owner needs to replace all the lower unit windows (uppers previously replaced) to mitigate the issue.

I am looking at this property tomorrow. My concern is that there may be future risk of lead abatement that will be needed.  

Does anyone have experience with this?  Should I stay away from this property?  The current owner is taking the proceeds from the sale and will replace the windows after closing as she is cash poor right now.  

At best, I'd proceed very carefully and get a company in there prior to close to make sure what your exposure for future costs and mitigation are.  Additionally, I would suggest an escrow holdback for the windows replacement, so that you are certain that that work get's done.  What's to keep the current owner from closing and taking her proceeds and disappearing, leaving you with all the windows to be replaced?

Let me start by saying that I don't know WI lead laws. You'll need to look them up.

That being said, if this were here in MA there would be a Lead Inspection Report. If you get me a copy of that, I can tell you what will need to be remediated in order to get a lead certificate. I highly doubt it's just the windows that will need to be addressed. Also, if this house were in MA, the windows (and other work) would need to be done by a licensed deleader, not a regular window guy. 

It could be more than just the windows that contain the lead. Proceed carefully and get a copy of lead disclosure from seller. If you are seriously considering it, you can have someone do a type of litmus lead test.

I personally would consult an attorney, because this could be a very litigious situation. You mention that one child is already experiencing problems and that knowledge alone loses the hypothetical landlord neglect case against you. The odds of it just needing replacement windows is slim to none and therefore could be an expensive fix.