O'Fallon is largely a military community, as it is the home of Scott AFB. Belleville is older, the county seat of St. Clair County. It has a town square and does have quite a few fairs, etc.
Here's the Wikipedia entry for the "Metro-East", which will give you a quick overview of the wider area:
http://en.wikipedia.org/wiki/Metro-East
For fairs and festivals, the St. Louis Post-Dispatch publishes a guide every May. The online version (STLToday.com) only covers things that are upcoming, but if you contact the public library in Belleville (
http://www.bellevillepubliclibrary.org/) , I'm sure they will be able to send you copies of the Metro-East pages for the entire summer.
As to private schools, do you mean religious affiliated, or independent? AFAIK, there are no independent private schools in the Metro East, though there are several outstanding (and expensive!) examples on the Missouri side of the river. This list shows the private schools in the Belleville area:
http://www.privateschoolreview.com/county_middle_schools/stateid/IL/county/17163
For Catholic Schools, you should check the Diocese of Belleville, which covers the entire Metro East.
Something about the tone of your post makes me think that you might be VERY serious about children's musical theatre. At that level, most folks in the Metro East would commute to St. Louis to put their kids into programs at COCA (
http://www.cocastl.org/) and audition for the Muny Kids.
(http://www.muny.org/content/view/63/43/) MUNY Kids is
very selective and serious; many of the kids accepted go on to professional careers in theatre.