We do not exactly know how long it takes for a child to grow up but any children you have will only grow to a young child, they cannot age beyond this stage to become a teenager, adult etc.
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In Fable II your baby (or babies) will only be able to grow to a child after you have come back from the Spire. From that point any children you have will become a grown child from a baby after one in game day and you must have left then re-enter the region to see the transition.
In Fable III it, again, takes approximately one in-game day for the baby to become a young child and you must leave then re-enter the region to see the change.