1) Homeschooling starts in Wisconsin at age 6, not 5 1/2. Your child is still kindergarten age and therefore is not subject to compulsory schooling laws. Do not let anyone bully you about this! At the very least, I would ask the Guardian why she is insisting on this when kindergarten is not compulsory in Wisconsin.
2) Here is a link to the Homeschool Legal Defense Association and their page on Wisconsin law - they are an extremely helpful organization but are fundamentalist Christians so if you do not meet their criteria they might not be willing to help you http://www.hslda.org/laws/default.asp?State=wi
3) You MUST contact the Wisconsin Parents Association (WPA) for help in this. However, they are a volunteer organization and as such are slow and sometimes hard to reach. However, they are experts in working with Wisconsin laws concerning home education. http://www.homeschooling-wpa.org/
4) Unfortunately, when a divorce or separation is being argued over with regard to a child's education, things become very complicated. You have left this very very late - I hope you are able to get enough support and information in time. If there is any way to delay a decision so you can get all the advice you need, I am sure that would be helpful!