With kids that young I would do St. Louis.

Their zoo is actually a happy place The animal habitats are outstanding (better and more spacious than San Diego) and it is free. Tha St. Louis zoo is by far the nest zoo in the country.

The Magic House

Wabash Pacific Railroad (take a picnic)

Grant's Farm

City Museum

Lone Elk Park

Tyson's Wolf Sanctuary

Brewery tour

Hannibal Missouri and a rivercruise

The Mid-America Six Flags is very nice

Museum of transportation

Museum of westward expansion/Arch

Historic St. Charles