Thank you very much Don!! It works well. I am happy.
I just added the argument 1 with the cloneNode function to get the child nodes.
You are right, <fixe> is not the root element of the file.
I should have mentioned that.
by the way, when I remove some nodes and then when I use toString or serialize, the xml is not really pretty. Is there a function to remove unneeded \n and to format the output with the good indentation? (a kind of "pretty print" with xmlspy for those who know this software)