What about intellectually handicapped children with potentially normal lifespans whose cognitive capacities will never surpass a typical human toddler or mature pig?

I'm not sure what this would look like, actually. The first thing that comes to mind is Down's Syndrome, but the impression I get is that that's a much smaller reduction in cognitive capacity than the one you're describing. The last time I considered that issue, I favored abortion in the presence of a positive amniocentesis test for Down's, and I suspect that the more extreme the reduction, the easier it would be to come to that direction.

I hope you don't mind that this answers a different question than the one you asked- I think there are significant (practical, if not also moral) differences between gamete selection, embryo selection, abortion, infanticide, and execution of adults (sorted from easiest to justify to most difficult to justify). I don't think execution of cognitively impaired adults would be justifiable in the presence of modern American economic constraints on grounds other than danger posed to others.