Answer:
Some children are too poor to pay their school fees.
Explanation:
In the original sentence, the use of "so" is an adjective that described the condition/ quality of the children's life. Added to that, the pronoun "they" is used to refer to the 'students' and also give the effect for the cause given.
To change the given sentence using the adverb "too" before an adjective without a noun. This will also enable the sentence to be made shorter, thereby making the use of "that they cannot" redundant.
Thus, the new sentence will become
Some children are too poor to pay their school fees.
