When it comes to knee guards, I found that an adult size elbow guard makes a great substitute for small children. We were able to save a weekend of riding by having my six year old son wear my size medium Fox Titans on his knees. He rode for hours over the course of two days with no comfort issues, and the smaller cups actually provided a better fit for his diminutive legs. The forearm and upper arm straps do a good job of keeping the guards in place, and they fit easily into his boots. His other knee guards, which were the smallest size available, were still a bit too big. It was difficult to get the top of the boot to completely close around them, and they just did not fit well.
If you decide to try this idea, make sure the knee guards are the style that have detachable, wrap around straps - these can be adjusted to fit comfortably without cutting off circulation. I would be reluctant to try this with the pullover sleeve type elbow guard as they cannot be adjusted, and would have to fit perfectly in order to be comfortable.