Petzl OUISTITI Kid's Harness & PICCHU Helmet

The OUISTITI seems less inclined to get twisted than some full-body harnesses - or even some adult harnesses come to that. If it comes out of your bag tangled, just grab the rear tie-in loop, give it a shake and it falls into shape. Putting it on is intuitive enough that it can be done unsupervised - it'd be pretty difficult to don incorrectly.