Both editions of the 1689 LBCF I have do not have “Elect infants”, but only “Infants”. Sam Waldron, in his Exposition, says,
“The questions over the meaning of the phrase, ‘infants dying in infancy,’ are a bit more complicated [as compared with those “incapable” –SMR]. In the Westminster Confession the word ‘elect’ is present, while it is deleted in the 1689 Confession. Its deletion does not, however, materially change the meaning of the phrase. The phrase, ‘infants dying in infancy,’ does not assert that only some infants dying in infancy are saved. It does not exclude that possibility, but it does not assert it. It does assert that at least some infants dying in infancy are saved. That is all it necessarily asserts.....
The fact is that the Bible is silent on this issue. It would have been much better, therefore, for the Confession simply to say nothing at this point. For that, I am convinced, is precisely what the Bible says.” (pp. 149, 150)
I am assuming that the only reason that this is an issue is because some come to the conclusion that since the infant never had the chance to 'believe' that it misses out on salvation. I cannot see any other reason why someone would be led to 'doubt' an infant's salvation.