My husband and I each have a moderate to small sized wardrobe. However, we only bring out about 3 outfits per season and wear only those clothes until the weather changes or we are tired of them for the time being or they are ruined. So we live with a very minimal wardrobe on a daily basis.
The benefits to this have been:
We do our laundry a few times a week, as soon as one or two small loads has accumulated. We never let dirty clothing sit around, so staining and clutter is less.
We tend to really value and notice each and every item that is is current circulation. Instead of putting a soiled shirt in the laundry, we tend to spot wash immediately, knowing that there is only one other shirt available for the next day. Same with mending.
We tend to want to wear out our clothing, because it's less attractive to pull out a "new" shirt from backstock than to just get all the life possible from what is already in circulation.
We have relaxed standards about what is acceptable. I don't have any problem with a slight stain on clean clothes on a clean person. Same with mended clothing. We don't aspire to anything and don't believe that people should be judged by their clothing.
We buy the very best quality clothes we can afford, even if it means getting them slightly used. Silk, linen, wool, and good synthetic blends do not stain as much (and are lower in pollution and water use) than cotton. We don't have much cotton clothing.
Although I espouse natural products in every way possible, for stains a spot wash with a small amount of standard laundry powder (without bleach) is a very good treatment.
I don't buy light colors and solid colors that show stains quickly.
Since you are a mother of a young child, I expect that the lifespan of your clothing will be much less than mine, since I have an "empty nest." I think that drawing from a backstock would be a good way for you to begin to address the issue.
I also feel that children, even young children, can be made very much aware that clothing is an essential and costly resource that needs to be cared for. Your son could learn to change into play clothes when he feels the urge to get active outdoors. I don't see anything wrong with dressing him in playclothes if he is going to a playground, or explaining that if he is going out for lunch in nicer clothing that a playground visit isn't appropriate for a young gentleman out to lunch: play is great and can definitely happen after his luncheon activity is over. This isn't really about the clothing, it's about learning to self-regulate.