"This book is about a woman, her husband, and their child."
Translation:Tato kniha je o ženě, jejím muži a jejich dítěti.
The sentence is talking about 'their' child, therefore: jejich (with short i.) This pronoun is not declined. (Yay!)
Jejím (with long i) is locative singular form of její ('hers')
Jejích (with long i) is locative plural form of její ('hers')
Pay attention to accents ;)