How to close the on-screen list of process My kids played with my Nexus 7 and since then, I have a list of what I think is process on the upper right corner of the screen.

The first line is three numbers, like 4.38 / 4.7 / 4.77 and a colored bar.

And then, a list of process(?)
com.android.systemui
system_server
mpdecision
...
How can I close all that ?
It sounds like your hunch is correct - it's basically a small process monitor. Go to Settings -> Developer Options and uncheck the setting labelled "Show CPU Usage" to disable it.