Child employment laws mean that every child has to get a "licence" to work, which takes several weeks to organise.
Also, arrangements have to be made for schooling on-set.
This means that no last-minute extras can be accepted.
How to become an extra
For the Harry Potter film, both specialist casting agencies and local schools were used to find extras.
According to Vanessa, the early starts (6-7am) and short working days made it sensible to use children who live nearby.