Since baptism is administered by fallible men, they do not always infallibly judge of the truthfulness of the subject's confession. Many are baptized whose profession is spurious, but if elders waited for absolute judgment-day certainty, they'd never baptize anyone. So they operate to the best of their judgment, leaving omniscience to God. Ideally, only those who truly are regenerate should seek baptism, but we do not live in an ideal world. We operate by the light we have now: God will judge righteously at the great assize. As for the elect children dying in infancy thing, RBs believe that God saves them without a baptism at all! Just like I'm sure you believe that an elect child dying in the womb is regenerated even though his little corpse never gets baptized.