Depending on how you use "Webkinz," using the (trademarked) name of another's products to sell your own could be deemed to trade off of its reputation and goodwill to attract customers to your offering.

If that's the case, you should definitely seek permission from Ganz. Speak to an intellectual property attorney to determine the appropriate course of action for you; from there, your attorney may be the one to contact Ganz's counsel to discuss the situation.