Pornography laws are a relic of the Victorian era, and not based on any science.
Research that can conclusively determine the effect of pornography on children is hard to come by due to the resultant ethical environment. Most research depends on self-reports: Surveys ask adolescents how much pornography they have been exposed to, and attempt to correlate this with other variables such as personal attitudes about sex. Such surveys are subject to a variety of memory and other biases affecting accuracy, but more importantly, correlations fail to establish cause and effect - that is, it is equally likely that pornography consumption affects attitudes as it is that pre-existing attitudes affect pornography consumption.
Nonetheless, a number of reviews of current literature on adolescents conclude that there is an association between consumption of pornography and certain negative attitudes about sex, and promiscuous and/or risky sexual behaviour. The reviews acknowledge that the data contains much contradictory evidence however, as well as failing to establish cause and effect.
UK Children's Commissioner's report: "Basically... Porn is everywhere"
DOI: 10.1080/10720162.2012.660431: "The Impact of Internet Pornography on Adolescents"
Research on pre-adolescents is even more scant. A more conclusive effect is seen between exposure to depictions of violence and negative effects on attitudes, behaviour, and possibly sex offences. A landmark study from the 1960's (no longer ethical to replicate) demonstrated that children do role model from violent media. The reviews listed above extend this finding to attitudes and behaviours of adolescents exposed to pornography depicting violence. Evidence linking this to criminal offences in adulthood is still controversial however.
It so happens that the vast increase in availability of pornography through the Internet coincided with an overall reduction in physical sex offences, though again, establishing a cause and effect is difficult. No doubt that as pornography has become much more accessible to children, a large social experiment is being inadvertently conducted that might shortly lay this matter to rest.