Parents who take their children out of school for a week or two, without permission, are usually only fined (typically �100 per child, although the fines can actually be imposed for every day that each child is absent).
Parents who take their children out of school, without permission, for longer periods are likely to be prosecuted. Such prosecutions usually result in hefty fines and/or community sentences (such as 200 hours unpaid work) but in a few cases parents are sent to prison (but usually for only a few weeks).
Chris