Bonus, this can also be used to remove YouTube and other kinds of ads as well as all the phone-home "analytics" if that's your thing.
-- Edit --
Some options:http://www.thekelleys.org.uk/dnsmasq/doc.html + ToDo's via Google
https://github.com/apankrat/dnswhisperer or more generally https://github.com/search?utf8=&q=ad+blocking+dns
There are also 3rd party services that provide this function. They have obvious privacy issues, but for a kid's iPad it might be an option.