That the bodies which belong to 5th children are put through a more rigorous pre-spirit filled qualifying test, and must be made of a more durable fabric than the average every-day body.
... the bodies must pass extra tests of being stretched, squeezed, squished, kissed, grabbed, stickied, and just plain used and abused.
I am convinced that the spirits which come to those over-tested bodies are also put through some pre-life qualifications themselves, including extra doses of patience, long-suffering, smiles, and unconditional love.
Otherwise, there is no way that 5th children (or the mother of 5th children) would ever live to see their first birthdays with any sort of sanity in tact.
... and that's all I have to say about that.