Sorry for the late response, but I was struggling with the same question for quite some time and maybe my experience helps someone. The plan was to find one device which supports Kodi/TVH, Netflix and Amazon Video and it seems like there is no Gadget out there which can do this. From my findings, I would suggest one of these four options:
1) If you do not need Amazon, just wait for Kodi v18 as it has native Netflix support. You can also use the Alpha Version of Leia already, but be aware that your family probably doesn't like to be Alpha testers and that a version shift may have negative consequences... (trust me, I know what I'm talking about ;-) )
2) Use two devices - one for Kodi (e.g. RPi or Odroid C2) and another for Apps (e.g. Amazon Fire TV or Mi Box).
3) Build or buy a HTPC on Windows 10 Basis. It'll do it all, but when it comes to remote control it can be a mess.
4) If you are patient and can live with some drawbacks - go for a XBox One S. XBox status thread: https://forum.kodi.tv/showthread.php?tid=326228
Personally, I decided for option 2) with an Odroid C2 and a XBox One S. The XBox basically can do it all and is made for use with a TV. MS just released the XBox One X, so there is a good chance to get it for a good price. It's just a matter of time, when the Kodi devs solve the problems and Leia becomes stable. While development is still in progress, I stick to the C2 with LibreELEC for TV and use the XBox for Netflix, Amazon, Youtube, Twitch and additionally can start a game when I'd like to (man, it's a console!).
If you prefer to buy a Mi Box 4k be warned that Xiaomi's support and update politics is just AWFUL. The box is still running on Android 6 and the latest security patch is several months old. It feels like they unofficially dropped this box. There is also the WETEK Play 2, an Android box which runs Kodi and the Google Playstore, but since they modified the Android OS it's not an Option for me (same with Fire TVs).