Sorry for the late reply.
1. Make backup of the Descr_strat for the selected campaign.
2. Open the strat and add a new character record at the bottom of the factoin`s section.
3. Assign him/her to a family. In my case I removed the oldest son of Phillip V and added our Alexander.
4.
NB: You should make sure the parents are old enough to have a child of that age. Since Phillip and his wife is around 24-ish, I increased the age to 43 for both parents, since the parents must be at least 16 years older than their children.
5. Start a campaign and check.