Can I get a refund on two applications my 5 year old son downloaded totally £138?

Purchases are considered final but you can try the 'report a problem' page to contact iTunes Support and see if they will refund or credit you : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

If they were downloaded on an iOS device then to try and stop it happening again you can use Settings > General > Restrictions to require your password to be entered for every download, and/or to hide the App Store ('installing apps') and iTunes store apps