Yea I think the full armour trend comes more from the mom department, its deffinatly overkill at that age in my opinion. Second I've seen moms fearing about trying to push their kids into adult sized armour because its protection is better, its just dumb. If the armour doesnt fit, the armour is no good. I tried explaining this to them but there was no chance I got through.
As a kid I think the best investment is in a good helmet first and foremost get kids used to wearing the full face all the time ASAP. I cracked my giro madmax helmet as a 12 year old and my dad without a twitch just went out and bought another one right away, it was a good investment.
Second get some shin pads. Kids dont quite shrug off the big pedal smacks to the shins quite as well as some of us have become acustomed to doing.
That should be good for now maybe look into elbow pads on the way for the older one.