Check out the Double JJ Ranch in Rothbury. GREAT place for all ages. We have the same issue with teens and a little one. Everyone has fun there.
The dunes at Silver Lake, just north of the Double JJ, are awesome, too. We've stayed at the ranch and at Silver Lake and have been able to combine both in the same vacation.
Michigan's Adventure is nice, too. Not so exciting for the older ones who spend lots of time at Cedar Point, but great for the little ones. The water park is better than the amusement park side to keep both ages happy. We stayed in Muskegon on that trip, so we did the great beach/amusement park vacation in one trip. Muskegon has beautiful beaches.
The Lake Michigan shoreline is beautiful! Have fun!