So basically, "por favor" in Portuguese is the same as "por favor" in Spanish?
When you have kids, it's everyday an horror scene. Personnally, I can use it everyday...
Being Portuguese and speaking other Latin based languages the expression as stated is in fact correct.
I don't think anyone is disputing whether the expression is correct; it is the translation that is in dispute. "Please, no" is very clumsy English and would probably mark you as not a native speaker.
I agree, "No thanks/thank you" is much more natural in English. We might say "Please, no" under duress, but what we really need to know is whether the above is the COMMON Portugese phrase, i.e. when offered a drink or a sweet.