When accompanying children (in my case roughly in the 5 to 9 range) on a moderately busy (by rural standards) road on our commute to their school I'm sometimes ahead, sometimes beside (at least the smaller one) and sometimes behind.
What is best practice?
Bearing in mind:
ahead of them I can control their speed and demonstrate to them safe cycling
beside is good, but this only takes care of one child (I'm cycling with two)
behind them I can see what's going on
The third option is what I tend to favour most.