Thanks all for the advice. I cleared the eeprom and pushed the sketch again. The arduino does get a new device_id so that worked. After unplugging it from power and putting it back on I got the same message however and no readings.
I found however that another sensor I have that is working and sending readings also gives a 'child_id already exists warning' when I powercycle it. So now I am thinking that the warning is normal behaviour when an already registered sensor comes back online.
Than I compared the serial output in de log parser (amazing what a handy thing that is) with output from a working sensor.
I noticed that everything looks the same upto the the line
My not working sensor gave no more output after that but the working sensor continued by sending a reading and going to sleep.
So I suspect there is something wrong with the sensor or sketch causing it to never correctly initialize.