The two problems with putting smaller children in climbing harnesses is that they are very top heavy and have very thin waists. This first problem means that they are likely to tip upside down if they fall off, while the second means that if they do tip, they can fall out of the harnesses. Designed to take kids up to 40kg (and most likely in the age range 4 - 10) this fully adjustable model helps solve those problems by being a full body harness which grips over the shoulders and keeps the tie in point higher to avoid tipping.
The Vision Kids has Ziplock buckles for simple quick and safe adjustment, more padding than most, and a great color scheme to encourage the little bears in your life to get going. One size.
Note: I have daughters aged 5 and 8 years old. One is cautious, the other...well let's just say her favorite word is "Weeeeeeeeeeeeeeeee" without any consequence for injury. I chose this harness so that I can teach them what an upside fall is and why to avoid it.