That's a slow time of the year, but weekdays will be much slower than the weekend. Even Saturday or Sunday won't be super busy though, just arrive early. I was there last week on Tuesday and we practically had the place to ourself in the Sesame Street area. I'd say if you watch the show, visit all the characters, ride every ride, and play in all the net areas, you can probably get everything in Sesame Street done in about 2 hours on a slow weekday.
There are also a few kids rides in the Timbuktu area (3 rides). If you like animals, check out the Edge of Africa area and take a ride around the park in the train.
Just a tip, if you get to Busch early (around open time), ask for stroller parking. There are only a handful of spots, but it will allow you to park across the street and avoid the tram.