if we're talking cost effectiveness:
the early intervention of places like sure start can encourage parents to parent in a more productive way, which can help to ensure that their children do not get involved with petty crime, alcohol or drug abuse, which can in turn ensure that they parent effectively.
Getting involved in helping and empowering families at an early stage can be supremely cost effective in the long run - keeping surestart centres open now may save millions of pounds in court fees, prison sentences, drug rehab, NHS fees etc. in the future.
Anyone unsure about the feasability of this can read Ian Duncan Smith's paper on early intervention. (yes, that IDS)