Number of kids is inside general flags like this for example flags.GWYN_PREG_NUM_KIDS (flags.pregkey_NUM_KIDS).
So doing something like flags.GWYN_PREG_NUM_KIDS = 10 - changes number of children with her.
Now the problem is that nursery bunches up diffrent pregnancy results together (depending on the race). So for example Orcs children are Arona's (when she is mom) + Arona (when she is dad + pc mom) + Hretha + Infrith + Ylvid's (only PC father) + generic orc (pc mom + generic orc). Align it all is not worth the effort imo.
Edit: Ylvid + Arona children do not go into nursery there is separate flag for that pairing (as far as code sugest).
If you want to tinker with it:
Code:
PregManager.nurseryPregnancies() - this gives list of pregnancies that actually goes into nursery (as not all goes there)
PregManager.nurseryPregnancies().filter(obj => obj.childRace === "orc") -> this gives you list of pregnacies for a race.
Ofc if your PC is only mom or only dad you need to pick and choose which are even present in your save file (above command checks only for race)
Then you take "key" and go with flags."key"_NUM_KIDS = X.
I'm sure you can somehow chain it more and get one command to changed them in for or foreach loop but im too lazy to do it.