no longer的問題

若然我想話過左今天,你將不再是一個小孩.

- you will no longer be a child tomorrow :
This is a correct sentence structure, but the Chinese way you are saying implies that "you cannot be a child anymore or you cannot act like a child anymore (你將不再.....)
Therefore the most accurate translation for your sentence is :
After today, you can no longer be a child. (This is what most mother will tell their kids on their birthday when the kids get older).
or
After today, you can no longer act like a child. (This implies the same meaning, the the one being told cannot act "like a child")

- you will be no longer a child tomorrow. (This sentence structure is wrong because you have to put "no longer" between "will" and "be", which makes it changed to the same sentence structure as your first sentence.

想話過左今天,你將不再是一個小孩.
1.唔可以寫成you will no longer be a child tomorrow,
2.亦唔可以寫成you will be no longer a child tomorrow

1.句子結構正確，但意思唔清楚。因為過左今天after today唔係淨係明天tomorrow，還有後天day after tomorrow等等....

所以要寫成:

After today, you will no longer be a child.

2. you will be no longer a child tomorrow
--句子結構錯了--no longer好似not一樣要放喺will同be中間。
--tomorrow也是意思唔清楚。