The Frank family chose to hide together because they would rather be together than apart, obviously. I do agree with Bettelheim that they should have devised some sort of escape plan in case they were found. They should have had some sort of weapon to detain the SS soldiers for some time in order for the children to get away safely. It would have been a smart idea to make such a plan, but they believed otherwise.
I do not agree with what Bettelheim said about it not being desirable to want to continue living as a family even with all the evil going on around them. The Frank's found it to be a desirable lifestyle, and its what they chose to do. Just because Bettelheim found it to be undesirable and others may have found it to be undesirable doesn't mean the Frank family also did. By hiding out as a family, they were not ignoring the threat of the Nazi's, they were doing what they though was best, which was be together for as long as possible.
Had I been a Jew living during this time period, I would have wanted to live hiding separate as to have more chance of survival, but that's me saying that now. I don't know what would have been worse, dying with your family, or dying without your family. I can understand the Frank family wanting to be together as long as possible in a time where evil was all