I had asked my vet about sedating Starr when we were planning a trip and he said it was downright dangerous to do that. There's no way of knowing how they'll react to the drug, and they'll be trapped in the carrier where you can't help them
The altitude changes can also affect how the meds work, so they could end up with a very sick baby.
I've heard that rescue remedy is great for this, but have no experience with it.
Good luck to them!