My guess as to why they did not travel back to this timeline is because they are there already as children and you cant be in the same time line in two different instances.

The 1974/1979 problem aside, it would also explain why Charlotte's body didn't travel with Daniel.

i dont think they(ben,sun, sayid) TT at all, they crashed on the hydra island after the pilot and O5 flashed out of the plane...

That would be a great theory.
If Sun's dad was involved with Dharma, she could be there visiting or something.

My guess as to why they did not travel back to this timeline is because they are there already as children and you cant be in the same time line in two different instances.

Sawyer and Locke sure were able to exist twice at the same when they saw the light from the hatch and Sawyer witnessed Kate help Claire give birth to Aaron.

Locke most certainly also existed twice at the time of the outrigger shooting.

Except they absolutely could be in the same timeline twice. One example is when the Lefties jump and Sawyer spies Kate helping Claire give birth in the forest. Sawyer exists in that timeline, all the losties do, and yet they all jump to that timeline thus existing in two different spots in the same timeline.

i dont think they(ben,sun, sayid) TT at all, they crashed on the hydra island after the pilot and O5 flashed out of the plane...

I don't think Ben was supposed to go back to the island in a philosophical sense, as he did not flash out safely, in one piece. However, his presence on the island is critical to the story line.