Youth pad set from Trinity. comes with wrist guards, probably not usable when riding BMX, but the elbow and knee pads are gonna stop some injuries and keep your young one shredding for sure. 

1 x set wrist

1 x set elbow

1 x set knee

Size Guide - Measure 10 cm above the centre of knee cap your leg circumference. (around leg)
Size guide is for the knee pads

Youth S/M 20-25cm

Youth L/XL 23-30cm