Translation:The child must not go swimming tomorrow.
Because can't means both he/she is incapable and he/she is not allowed.
In this case the Vietnamese is clear and distinct whereas the English is vague and nebulous; English is the tougher language to learn. The only language I know of that might be considered more confusing would be Mandarin, if only because each character represents a whole concept or word, and if you don't know the character then you can't sound out the word to decipher... At least that's what I've been told.
The meaning of the sentence would change if 'không phải' was used: "The boy does not go swimming tomorrow." The distinction between the two is that 'không phải' expresses the negative of an absolute and 'không được' expresses the impermissible action of the subject. For instance: "Tôi không phải là con chó" "Bạn không được ăn trước khi anh đó" These are just my thoughts that I tried to organize. These may not be 100% correct, sadly.