Greg responds...

We haven't stopped. Amistad, for example, was played by a child actor. We're just admittedly inconsistent about it, for a variety of factors. Usually, the reason is budgetary. If a child character only has a couple lines, and we think we have someone from the existing cast - in that specific episode - who can cover those lines, we may elect to save some money, so that we can spend it later on another actor. And then once we've cast someone, we rarely recast, unless (a) we have to because the original actor is unavailable or (b) the character has aged so much that the original actor is no longer appropriate.