I don't know the correct polygon so I can't fix that, but I would like to check if it is correct to remove the country children (and indeed I don't know if I would have privileges to do this)
Thanks for pointing this out. I have removed the countries as children. I have also renamed the feature to Parc National du W. There is a second feature which only covers the part of Nigeria:
http://www.geonames.org/6980619/parc-national-du-w-du-niger.html I have added this feature as a child of the entire park.
There is no specific boundary polygon for this feature, the displayed polygon has been built from the hierarchy as union of the three country boundaries.