iTunes Customer Service Contact - http://www.apple.com/support/itunes/contact.html
The only link I have with mention of a phone number would not work from the UK. I am surprised it is left active since iTunes Store is noted for not having any phone number and only reachable online. iTunesStoreSupport@apple.com
iTunes Store: About In-App Purchases - http://support.apple.com/kb/ht4009
iOS: Understanding Restrictions - http://support.apple.com/kb/HT4213 - "You can enable Restrictions, also known as Parental Controls, on an iPhone, iPad, or iPod touch to prevent access to specific features."
Children should not have access to your iTunes account password.
You should also consider removing credit card information from your account so no unconsidered purchases can be made by anybody. iTunes Store: Changing Account Information - http://support.apple.com/kb/HT1918