We just bought Platinum Passes so we can spend all summer at any Cedar Fair park
If your kids like thrill rides, I would say 2 days. And try to go on weekdays. Cedar Point's lineups in the summer can be really nuts for thrill rides and there's no FastPasses. In the summer, for the major rides, expect to wait 1-2 hours for each on average. Some rides i've heard of up to 3 hr lineups.
That said, Cedar Point is your thrill ride mecca and has the absolute best collection of rollercoasters in the world.
My dd9 is just getting into thrill rides so we've been going on some big ones - but not the top 4-5 big huge ones yet. My ds8 does not like thrill rides and finds enough to do with the carnival type rides, smaller coasters, and tame stuff like antique cars. I really like that there are kid rides scattered throughout the whole park. It really helps families not get split up too far.
If you do the waterpark, there's a separate admission fee. They do sell a 2 day ride and slide ticket.
You can do all the big rides in one day - but if your crew really like thrill rides, then they'll want to re-ride lots and the 2nd day is good for that. If you like shows, they have a good ice skating show as well as a few other shows. They also have Starlight Experience which is a lighted walkway (maybe a tiny bit like Osborne lights?) and fireworks on some nights.
Have fun! It's no WDW - but for thrill seekers, there really is no better park.