Taxi in Tokyo

If you can't find a taxi on street-level (which is unlikely), you can download an app called "Taxi!!" and call your taxi from the app.
Here you can download the taxi app:
Japan Taxi - https://play.google.com/store/apps/details?id=jp.co.nikko_data.japantaxi (Android) and https://itunes.apple.com/jp/app/id481647073?mt=8 (iPhone)

Here's a list of other useful apps for your trip: 9 useful apps while traveling in Japan
https://planetyze.com/en/japan/tokyo/blog/9-useful-apps-while-travelling-in-japan

If Japan Taxi isn't enough for you and you want to be safe, there are other taxi apps such as:
Line Taxi - https://line.me/en/ (you download the line messenger and the taxi feature is in the app)
and Uber - https://play.google.com/store/apps/details?id=com.ubercab (android) and https://itunes.apple.com/en/app/uber/id368677368?mt=8 (iOS) although I should say Uber is expensive because you can only hire Uber Black.

Keep in mind that If you have a lot of luggage, you may need to hire more than one taxi for your family.

But don't worry, the train system is very good and if you use google maps you can find all the routes reliably, so don't worry =)