I did a search and i know a shield is the best but wonder if its over kill (price wise) for young kids. They mostly would use it for minecraft pe and the rest is netflix/youtube.
I did have some mxq pro 4k's but they are getting slow and cant run games. did look at a matricom g-box q3 wodering if it can do the job.
The MXQ Pro 4K runs the same chipset as the Mibox (S905x), so it should be able to run Minecraft in a tolerable enough way. You'll have to adjust settings so that it runs smoothly.
But you'll need to sideload it.
For young kids, just get them an Asus or Lenovo tablet. Minecraft is easier when its on a touch screen. If you have more than one kid, then they can play together via local WiFi. If the realm gets too big and slows down the host device, you can copy the realm to the official bedrock server software and keep on building.
Wanting to run Netflix rules out any but a few specialized legit AndroidTV devices. Have a look at the sidebar list.
If it's any consolation, you would need to sideload to the Mibox also.
If it's for gaming, namely Minecraft, you might want to go with a PS4 since it's $300 and can do streaming and blu-ray too. The Shield TV is around $160 and not as good for that sort of thing, but is incredible for streaming and other media playback. I would not attempt Minecraft on any device lower end than a Shield TV.
The shield is the only thing you listed that is android tv the other 2 are tv boxes that run a the tablet version of android that said a mibox s would work fine for your requirements