If you can find it, get hold of the Best of Hughes Aufray 2 CD set.

On it: Santiano and Le Port de Tacoma are shanties at least, Je reviens (les portes de St-Malo) maybe (though I'm not sure how traditional) and Aufray is very listenable for children.

(You can also find Dylan and Hank Williams turned into French on it and some 'real' French songs. For the last, I especially recommend L'epervier (it's traditional enough so the words are on the web, and is much darker and more dramatic than the usual school French songs. Not a shanty, but I recommend it because I was introduced to it as a kid and still remember it. (L'epervier il faut le dire est petit mais bien voleur...)