Like Ann-Marita, I just buy two pairs at a time, but in 1/2 sizes. My oldest is 8, and goes through about 3 sizes a year (outgrowing that is, not destroying... if we counted that it would be more like 5). My yonger child, OTOH, grows much slower and only goes through 2 sizes a year.
I read somewhere that, like your ears and nose, your feet always grow as you age... although it is only a tiny bit every year. I think that about 16 girls feet are about what they will be and boys age 18... but I could be very wrong