TV's Frink said:

I've been thinking quite a bit about how to present Star Wars to my daughter, although she is still several years away from watching it. I always assumed we would go 4-5-6-1-2-3, but now I've got to consider this 4-5-1-2-3-6 proposal instead...

Another point to consider, particularly for younger viewers, is the note on which you want to end.

In ROTJ, Anakin throws off the yoke of darkness and reemerges as a good person. The Emperor is defeated. There is a big celebration with music, dancing, and fireworks, and the heroes pose for a group photo as a father and teachers look on with pride. It’s a cathartic and uplifting conclusion to the series and belongs at the end, not buried in the middle.

By contrast, ROTS ends on a sad note in the middle of the story, a conclusion that is likely to be unsatisfying to younger viewers, and a less fitting conclusion than ROTJ even for older viewers.

Lucas has stated that his intent is that the episodes be watched in the order 1-2-3-4-5-6. (This is also attested to by the fact that the upcoming 3D versions will be released in that order.) ROTS was never intended as a conclusion the way ROTJ was.