>>For years the population had been growing and sending people to a new planet seemed logical, even though it was slightly scary.<<

This would benefit from editing for clarity. I suggest something like:

"For years, the population had been growing. Sending people to a new planet seemed logical, even though it was slightly scary for those making the trip."

>> clinging to their mum’s or dad’s <<

Delete both apostrophes; plurals don't use any.

>>Not only would it be good for the planet it would also be good for them, because they would be some of the first people to populate Thear, so they could make it their own.<<

This stack of clauses would benefit from separation, such as:

"Not only would it be good for the planet, it would also be good for them. They would be some of the first people to populate Thear, so they could make it their own."

>> Shaking her head Tabitha kept walking,<<

That needs a comma after "head" above.

>> “If I could have chosen this I think I would have done, <<

This raises some serious plot questions. Why isn't this being done on a volunteer basis, instead of a draft? Are people chosen randomly (in which case they're likely to get poor results) or based on specific qualifications?

I have to think there's some huge drawback beyond simple distance, because humanity has spent much of its history thronging toward new frontiers. Needing to hold "an America wake" for the emmigrants never stopped the flood.