The Petzl Simba Full Body harness is fully adjustable and specifically made for children.  It even has cool kids colors.   The Simba is fully adjustable for children from 5 to 10 years old and less than 88 lbs (40 kg) in weight.

• Petzl Simba - Full Body Harness For Kids
• Totally adjustable: Shoulder straps and leg loops
• Easy to put on: Bicolored webbing; black on the inside, color on the outside
• DoubleBack adjustment buckles
• Double chest tie-in point