I managed to get a nice long and low outline with my young horse just using a lunge line and rope halter. I have a great instructor who taught me exercises to do using poles, varying circle size and changes of pace. Used straight sides of school to travel allowing him to stretch and move to a different area so not to get bored. He soon figured out how to use his inside hind to stay balanced and really started to use his whole body effectively. His head and neck naturally lowered as he started to use his back and hind quarters more.. all achieved without any gadget or bandage or whatever else is out there these days.
I grew up believing that these lunge aids and gadgets would help my horses way of going and have used my fair share.. (including a pessoa!) but looking back all I ever achieved was a fixed outline and horses that hated being lunged! I had a 4 year break from riding while my foal grew up and in that time read countless articles and researched several training methods because when the time came to back my boy I wanted to do it right. After a few early struggles with ground work I realised I needed some help so I found myself a good instructor who has taught me a very different approach to training horses and I'm now a firm believer that you can achieve the same result in a much less intrusive way.. you just need to find the exercises that work for your horse.
I honestly wouldn't use anything on a youngster. I understand why side reins are popular but personally don't see the point, long reining will help introduce contact and there are so many simple exercises that will help your horse figure things out for himself rather than being annoyed by some strange strap.
Everyone is different and will have different opinions, I think training young horses is a lot of trial and error.. what works for one won't work for them all so I guess all you can do is take your time and see what your horse likes the best! There is also immeasurable value in a good instructor.. I'd be lost without mine!
Hope everything goes well whichever methods you use.. good luck!!