+1 for The ixs trigger in xs if you get the chance. Kids necks don’t need the extra weight of Full on dh lid, and they’re unlikely to be carrying the speed and momentum to warrant it.
It’s also one of the only adjustable ones so sizing is a bit more flexible. There’s a lot of paying big boxes if you need to try and return internet orders.
That said, another option is the phoenix Lazer plus. Because you can usually walk into Halfords and try for size before you buy. My 11yo daughter’s in her second one of those, a small which she’s had a couple of years – before that an xs for 2 years.
Boy had a 661 and was v happy.