Another vote for the BBBs from Kids Racing linked above. My son’s had a couple of pairs of these and they’ve both lasted a very reasonable amount of time given the use/abuse they get in MTB and CX. We only changed the lenses twice a year – yellow for the winter, and tinted in the summer – and that’s it!
If you have deeper pockets, the “Oakley Radar EV Youth Prizm Trail” (phew!) are superb glasses and the lenses are versatile enough for use in both sun and woodland conditions. You can pick them up for less than £80-ish every now and then, but make sure you are looking at the youth fit ones as they are the narrower version.