@JasperNLH 

I am assuming you have modem/router, otherwise you wouldn't be able to connect three Ethernet cables to it. With that assumption, what you want will work - with Deco mesh running in Access Point mode.

It is better to hardwire all Decos. Definitely better. When they are hardwired, they'll give you same consistent WiFi speed near each Deco node. When they run connected wirelessly, expect losing half or more of WiFi speed on device wirelessly connected to Satellite Deco vs. device connected to Main Deco.

Now, one important point to mention: parental control.

In its current state, Deco mesh does not have meaningful parental controls: they are too easy to bypass. Deco does not safeguard from "randomized MAC address" feature present in modern smartphones, it does not have whitelist to enforce only MAC addresses you trust to connect. You can't set default catchall very restrictive profile for unknown MAC addresses.

Which means, anyone who can figure how to enable "MAC address randomization" on their device will bypass Deco parental controls.

If you do need parental controls, I strongly recommend going with different WiFi mesh brand.