Question:
This is probably more of a Minhag question, than Halacha,
Where / How have Yidden kept records of their children births dates. For instance for boys its crucial to know exact day to know BM.
Obviously nowadays its easy to find out, but surely throughout the centuries Klall Yisroel must have had an established way of recording one`s family`s important dates.
Answer:
There were families that used to keep a record of all important dates in a special book or in the back page of a siddur. I am not sure why they are allowed to use the siddur for this, but it could be that if this was the custom, then it is considered as if the siddur was originally designated for this additionally use.
Best wishes
