You really need to live in Spain, as I don't believe you'd be able to enroll your children in Spanish schools unless they where private.
But if Spanish is necessary then the only realistic place would be the Minho region between Caminha and Moncao, where Portugal and Spain merge in lots of ways, although Spanish are reluctant to understand Portuguese, and of course the local Spanish dialect is Gallego so you'd need to insure they learn Catalan.
The East border with Spain has less access and less towns in comparision to the Minho.
Surely if you can get a Visa for Portugal you should be able to get a Visa for Spain?