7 days is quite a short time to fit all of those in, and have rest days. In terms of route, the most logical one would be to do Holiday World, Kentucky Kingdom, Kings Island, and Cedar Point. I would strongly advise doing Cedar Point last as otherwise you risk having the other parks overshadowed by your experience at Cedar Point.
What I would say is check the operating calendars as some of those parks will have mid week closures in May which would seriously impact your ability to do all the parks you want in the seven days.
In terms of the number of days at each park, I'd advise at least 2 days at Cedar Point (normally I'd advise at least 3, but given your limited time there 2 would be the minimum); Kings Island you could do in a day if it is open until late, but you may need a day and a half if the park closes before 7pm. Kentucky Kingdom is definitely no more than a day - I only spent half a day there on my recent trip. There are only really two decent rides there - Storm Chaser and Lightning Run, plus an ok wooden coaster. Maybe look at doing 3/4 hours at Kentucky Kingdom then driving to Kings Island for the rest of the day. I can't really advise on Holiday World having not been there.
The busiest park will most likely be Cedar Point, but May is difficult to predict crowd levels. for Kings Island and Cedar Point one way to get some idea is to look at what they're charging for Fastlane + on the days you want to visit - the higher the cost of Fastlane, the busier they expect it to be. The Fastlane is expensive, but it works pretty much perfectly with most rides having a Fastlane queue that takes you right to the station. If you're not sure before you go, you can judge thing when you get to the parks and buy Fastlane on the day if needed. I don't recall Kentucky Kingdom having a fastrack option, and I couldn't say about Holiday World.
As you plan to visit two Cedar Fair parks (Kings Island and Cedar Point), once you know how many days you plan to spend at both parks and add up the combined ticket costs. It may be cheaper to purchase a Cedar Fair Platinum Pass. If you did get the Platinum Pass then you'd also get early entry at Cedar Point, and discounts on food, merch etc.
Hope that helps at least a little