As of May 2018, parental control is available as a built-in feature of Alexa.
Designed just for parents and kids, FreeTime on Alexa brings all-new Alexa features and parental controls to Echo, Echo Dot, or Echo Plus, including:
- Time Limits – Set bedtime time limits to protect against kids talking with Alexa late into the night, or simply pause Echo devices for dinner or homework time.
- Parental Controls – Use the Parent Dashboard and Alexa app to choose which services and skills kids can use, turn off voice purchasing, and more.
- Block Explicit Songs – Both in the Alexa experience and FreeTime on Alexa experience, parents with Amazon Music now have the ability to filter playback of songs with explicit lyrics, and we’ll continue to add compatible music services to this feature over time.
- Activity Review – Parent Dashboard allows parents to see their kid’s FreeTime on Alexa activity, plus review their voice recordings in the Alexa app.
- Magic Word – The new Magic Word feature offers positive reinforcement when kids use the word “please” while asking questions of Alexa.
Educational Q&A – Kids can ask Alexa questions about science, math, spelling, definitions, or any other topic that piques their curiosity. Since Alexa is built in the cloud and always getting smarter, she’ll continue to learn new information every day.
- Alexa Speaks “Kid” – Alexa has age-appropriate suggestions at the ready, all kids have to say is, “Alexa, I’m bored.” Kids can also ask for knock-knock jokes, ask Alexa for a song, and more. The more they use Alexa, the smarter she will get.
- Household Communications – Gone are the days of shouting up the stairs for the kids to come down to dinner. FreeTime on Alexa allows for household announcements, calling, messaging, and Drop In within the home.
This is paired with the launch of Amazon's "FreeTime Unlimited" subscription service, which provides a large set of "child-friendly" media content usable on the Echo.
In addition, Amazon is launching the Echo Dot Kids Edition, which appears to be a normal Dot with a new case and free access for two years to the FreeTime Unlimited subscription service.
Previously, there was no parental control functionality; my previous suggestions are available here in case they may still be helpful.