With fiddler, I found that the relationship list name I used was not correct.
I thought I had to use the static name of the list (that we can find in the URL), but finally it seems that it's the display name that we must use.
When I put "Services Web" instead of "ServicesWeb" it works ok. So this problem is solved.
However, I have another question about SPCascadeDropdowns :-)
In the case of a multivalued child list, I wonder if there is a way to keep the values that have already be selected, so that we are able to select children which come from different parents ?