Childbirth?
Or... conception?
Well ,she did mention he was really small and frail and pale at birth, so I guess that's as much as her small hips can handle? ╮(￣▽￣)╭
Well, jokes aside, consider that those with dwarfism can give birth to non-dwarf babies. Conception isn't necessarily a problem, either.
The mother might be small, but she's still a woman.
That's the thing, this is the stereotypical harem with a possible loli-complex in my opinion. If she had dwarfism she wouldn't look like a kindergartener, she would look much different. However this is a harem and she is somewhat sexualized for a 42 year old like all harem female characters, even the little ones, are.
All I wonder is how the mom (that looks like a five year old) was able to have kids...(⊙…⊙ )