Here a few tricks to make the transitions easier:
Elastic Laces - You don't want your kid to have to tie shoe laces in the transition from swim to bike! There are two options. Shoes with Velcro they can just slip on. Or buy or make some elastic laces that you can replace the normal laces. A final tip. Sprinkle some baby powder in the shoe to avoid blisters. No time for socks!
Race Belt - Kids should plan to complete the triathlon in their bathers (i.e whatever they swim in). They do generally need to wear a number according to triathlon rules. The solution is to attach a number to a race belt and putting that on. Much quicker than putting a t-shirt on a wet body! You could probably make one with elastic from the craft store but you can also buy them online for about $10. May need to be modified for skinnier waists!
Most kids aged 6 and above should be able to handle the skills with a bit of practice.