Sawyer Point, on the riverfront, is always fun. Walking along the Serpentine Wall, and the playground can be enjoyed in almost any weather.
http://www.cincinnati-oh.gov/crc/pages/-5711-/
Eden Park is great fun. Krohn Conservatory, and the Art Museum are there, as well as Mirror Lake.
The Krohn Conservatory has always been big fun for my family, especially during the butterfly show {May-June}.
http://www.cincinnati-oh.gov/cityparks/pages/-3452-/
At the Art Museum, they do a little scavenger hunt for children that makes it fun!!
http://www.cincinnatiartmuseum.org/
Everything, afore mentioned, is free {donations accepted}.
Go to Union Terminal, to see great architecture, and some great museums. If nothing else, your toddlers will love the Children's Museum. The Cincinnati Historical Museum has a great train display, that rides through a display of Cincinnati. And the Natural History Museum is great!! Not to mention the OmniMax.
http://www.cincymuseum.org/
Our Zoo is one of the greatest. It has been voted the "most romantic". Maybe not what you are looking for, for your children, but a great Zoo.
http://www.cincinnatizoo.org/