一般词义的区分最好看英语的解释
child
1.a young person from the time they are born until they are about 14 years old
指从出生到14岁的人
如：The nursery has places for 30 children.
2.used for showing that the person mentioned is a child
指提到的人是一个不到14岁的孩子
如：a child actor
3.someone’s son or daughter of any age
不管多少岁,当指别人的儿子或女儿时,永远用child
如：All of our children are grown and married.
offspring
1.someone’s child or children
某人的孩子或孩子们（后代）
如：conflicts between parents and offspring 父母和孩子之间的矛盾
2.the baby or babies of an animal
动物的baby
如：The birds will see you as a threat to their offspring.
3.something that has developed as a result of something else
因为有后代的意思,那么引申为某事所导致的结果
如：Jungle is an offspring of techno and jazz.