Which Harness

Crewsaver make a really good Junior harness with no exposed buckles. Which is safer. My son finds it very comfortable. The life jacket thing is debatable. Some people wear the harness over the BA so the can get it off easier in an emergency. I wear the BA over the top of the harness and sometimes wear a rash vest over the top of all off it, to make it more streamlined and avoid snagging on things in the first place. Having the BA on top also allows you to take the BA off if you are trapped to swim DOWN and get off the entrapment on the harness. Talking about this makes it sound as if trapezing an extreme sport but I have never hand my hook caught in any thing in 20 years. So go with what is most comfortable!

At first, she might want to wear a bit more whilst learning to trapeze, especially if her helm is new to trapeze boats too- because there's a pretty good chance she'll be spending more time in the water...

I always wear my harness under my lifejack.  I find the shoulder straps feel more secure.  Looking for a harness - fit.  Try as many as she can, perhaps ask some of the other 420 girls if she can try theirs.  Don't buy a true nappy one as she is probably growing so it won't last for very long.  The Zhik one i like as it has no loose straps, but again - try some on.