I think that you need to consult with an intellectual property attorney before you invest too much time and money into this venture - there are just too many unknowns to answer the question here.
Query: "[w]ould I need explicit permission for this kind of derivitive work?
Answer. You want to sell a 'toy/widget" that looks identical to someone else's toy/widget and wonder if that's lawful? The short answer is clearly no. Without more facts no one can tell under what body of law it is that makes your conduct unlawful but, generally, the law of unfair competition (state and/or federal) proscribes such conduct. You would need a license from the company that owns the rights to make the underlying toy/widget.
The key to what you want to do, and why it's infringement, is that it's recognizably this original toy, and you're trading on that recgnizability to make your version of it. Without more ifnormation on what it is you're copying, it may be trademark infringement or trade dress infringement - probably not copyright infringement.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.