Designing a playground, classroom, an office, or hotel lobby is really not any more complicated than designing a living room or kitchen; it's simple if you stick with these guidelines: function, mood, and harmony.
In this case, consider first the function. Indoors or out, you want the room to allow the kids lots of mobility for creative play. Time was when classrooms for any age consisted of rows of chairs bolted to the linoleum or wood floor; today, education has progressed to an understanding that children shouldn't be strapped into place at all times, much as a teacher might wish for this by 2 p.m.
Particularly in a preschool, the function is for the kids to be able to play in small groups and pairs, and at times alone, so flexibility in furnishings is key. You want furniture that is of course kid-size, but you also want it to be easily moveable and light-weight enough so that the children themselves can move it around as necessary.
Think too of versatility. You want playground equipment that will allow for a variety of activities, from swinging to climbing to chasing each other around in circles. A preschool classroom has many activities going on in the course of the school day, from finger-painting to story time to snack to building. Low tables with smooth, easy-to-clean surfaces can serve as both dining tables for snack time and as work surfaces for art projects or puzzle making. And if the tables are light-weight enough, they'll be easy to move out of the way for nap time.
Next, look at mood. One common mistake in designing for children is to think that children for some reason want colors brighter than any adult can stomach, and big cartoon characters plastered all over. We tend to associate childhood with an upbeat, happy mood, but this doesn't mean we have to overwhelm the kids with colors so bold they can't think straight.
Instead, try for a mood of calm happiness, with lots of natural images and colors. What about a green, super-durable carpet, and blue walls with a mural of puffy white clouds rimming the top of the walls and scattered across the ceiling?
Living things in the classroom contribute to the feeling of well-being, and a big aquarium is always a hit with pre-schoolers, as long as there's an adult around who is willing to maintain it.
Finally, don't forget to consider the harmony. Beware of throwing in everything that could possibly fit in the room; just because a chair is designed for children, it won't necessarily harmonize with the other furnishings you have. Decide on a single look for the room, and stick with it. You don't have to give up harmony in design just because the kids are going to be tearing around that room. In fact, the room will stand up to their destructive abilities better the more harmoniously it's designed. Tip:
Don't forget the clean-up! Attach a paper towel dispenser to each of the walls of the pre-school room, so the towels will be handy in case of spills. You could even design a shelf with a bracket just for handi-wipes.