I've got a web filtering thing called Mobicip installed on DC's iPods - there's an Android version which I haven't got round to installing on DD's Nexus. There's a basic free version, with premium stuff for more monitoring/reports - & they have a newsletter and website which is pretty useful, including a page called "How To Setup Parental Controls On Android" - worth a look!
Also, have a look at a web site called "Be Web Smart" - some useful articles on there too.
I still haven't worked out how to control YouTube though. I monitor DC's browsing history and have Safe Search turned on in google on the laptop.