The rear ones are easy to take off : push the side button, and take off the head rest.
For the front ones, it's not the same : they are blocked at their top level...
How do I proceed ?
I have received new headrests with monitors that I would install for my children :banghead:

theres going to be a little hole on one side of the headrest retainer (the part the headrest post goes into)... once you find that you need to poke a pin or something into that hole to release the headrest

xtuffycorex, you saved my week-end :-)
Thank you SO MUCH for your help.