If a pregnant women added 50% for the unborn child, then I'd expect that a born child would be worth at least as much. Indeed, given that the mother dying at birth was not a too rare event, the very fact that it was already born would likely increase the value. Given that especially at the beginning, the probability of the child dying was about 25% in the first year, I'd expect another value jump after a year or so.
Children were indeed valued in the medieval time:
Children were of course valued to some extent, and while they may have been seen as useless, they were not seen as worthless.
There is some record of Medieval families taking every measure possible to recover their sick children, even though it certainly cost them a great deal.
At an age of ten to twelve you were considered adult. So adult value should apply.
So my suggestion would be something like:
- 3/5 of the adult value during the first year.
- 4/5 of the adult value between one and ten to twelve years old.
- Full adult value afterwards.
