One cannot expect children to understand these problems.

Children cannot be expected to understand these problems

Children to understand these problems cannot be expected

Children cannot by expected to be understood these problems

To understand these problems cannot be expected from children by one

All of the above

"Children cannot be expected to understand these problems."

Children cannot be expected to understand these problems. (Passive)

The sentence is in Simple Present Tense. (Active)

Modal (can) has been used.

The other options are incorrect because they either misuse the word order or grammar.