It's a common argument I hear from so-called "Libertarians' that rights require reciprocity. Rights have nothing to do with reciprocity. An individual's right stems from the concept of self-ownership. As in individuated consciousness that is aware of your own awareness, you own yourself and thus people do not have the right to initiate violence on you. Most animals do not have the same level of consciousness as humans but at the very least deserve the right not to be killed or physically harmed for the very reason that you would not want that done to you if you were in their situation.
Just because a child or a retard does not have the capacity and or awareness to understand moral principles does not give you the right to harm that child or retard. Such an act would be unconscionable regardless of any established social contract.