I caught DS1 on the iPad this morning googling 'naked horse' 

The images that came up were, as you would expect, a naked women and a horse. All quite White Snake video but still - not what I want him to see.
First he denied it, then he couldn't say why he'd googled that in particular.
How am I supposed to tackle this.
On the one hand, it's probably the same as us looking up rude words in the dictionary when we were 9 but the internet is a crazy scary place!
I have parental controls but not sure how these images got through or how they police it.
Or should I be more worried about his 'niche' interests!
Help!