I will throw in a vote for making reservations along the way, if that will fit your travel schedule better.
Sure being spontaneous is fun sometimes too, but being a young family with kids (like we are too) does require some predictability at times depending on your and your kids' preferences.
KOA's are nice but a little overrated IMO. They are usually $40-50/night but don't have a whole lot to do on-site, at least the ones around IL. Instead they are often placed close to major attractions, and that's their selling point. State parks in IL just can't be beat, when you come through here on your way north to WI. $20/night at most places, great scenery and trails at many of them. Usually no full hookups (electric only) but we've never needed anything else for 2-3 day stays. There is fresh water and dump station on site, just not at every campsite. Here a few IL recommendations:
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Pere Marquette state park near St. Louis (haven't stayed there yet but plan to this year)
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Shabbona Lake state park (same as above)
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Starved Rock state park, another link
here (nice campground, great park/lodge with Native American history & trails)
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O'Connell's Jellystone (Yogi Bear park, expensive but lots of things to do, pools, waterslide, paddle boats, etc.)
The last 3 parks are all pretty close to one another (within 30-40 miles I'd say) off of I-39 toward Rockford, IL before crossing into southwestern WI. In case that falls into your planned travel route. You would definitely need reservations for O'Connell's Jellystone.
Mapquest/Google Maps are okay but I'd highly recommend investing in an automotive GPS unit. You can get an entry level one (like a Streetpilot c330) for $70-80 or less now. That's what we have and I wouldn't be without it now, especially for spontaneous traveling. Hope this helps!