Исправьте ошибки 1)I know Ben since my childhood 2)Since when did

1). I have known Ben since my childhood.

Present Perfect is correct.

2). When did you start to work?

Past Simple interrogative. The word since is not needed here.

3). George has been the best student since he entered the University.

Past Simple is correct.

4). The children have been changed a lot in the last time.

Present Perfect passive is correct.

5). Mr. Brown was not at work for last week.

Past Simple is correct with the tense marker last week.