Great reading, thanks

.
Well I think that the first step would be to agree to disagree, i.e., convince them that we shouldn't be killed just because we... ahhh don't want to use the spoiler tag.... we don't do
that thing.
Then begin trade routes. Get the less morally psyched people to exchange things and to know each other.
Then kinda trying to make the point that perhaps their conscient whatevers are suffering, and suffering is supposedly bad, right?
And this is the point, really. If they do not acknowledge that their whatevers are suffering, either we could show them how that is indeed the case, or we could actually find out that no, they do not actually suffer for it.
The last case is the most interesting one. I'll stick with the former for a bit though, for it is easier.
If you can convince them that they are making their whatevers suffer while it is quite possible *not* to do so, ahem, our own example is evidence enough, then only a religious fanatism would prevent them to gradually change their ways. Another reason why religion is evil

(hehe).
This "gradual" approach would be, I think, the one that would bring the least suffering for all involved.
Now for the "former" case, where the whatevers actually *do not suffer*. Now there we would reach a connundrum, for one
could make the case that their own society is actually better than our own!! The reason? Well, if they didn't eat their babies, there would not have been enough food for anyone. So they have to eat them. If they chose only to get one baby, then the amount of consciousness that is enjoying life is a hundred times shorter in that short timespan than what would have been otherwise.
If one makes the case that it is "better" for more consciousnesses to enjoy life, then their life cycle is probably maximizing just that.
In such a case, I'd recommend to try to "get along" with them and deprive ourselves from getting too much contact with that kind of reality, just as we don't like to get in contact with other people's defecations and whatevers.
Idk, only my first thoughts on the issue.