separation

As-salamu `alaykum.Some non-muslims criticize Islam  and Islamic Law in slavery (in history) as:1) According to Imams a female slave could be separated from his child while it is 7 years old . (They say the child should  be kept until he/she was 18) 2) Even according to Imam-? Malik the child could be separated while it cuts a tooth. (They say this is too early. The child should  be kept until he/she was 18) 3) The situation for the slave father was even bad. He could be separated from his child while it is under 7 year age. (They say also the slave father and his child should be kept until the child was 18) 4) The Prophet Muhammed (S.A.V.) could say that the child should be separated  in year 18 from his mother  and father. This could be a hadith and solve the problem in Islam in the history and very easily, in just one sentence. But this did not happen.So in general, they criticize Islamic perspective to  Slavery by separation of child from his parents. They say that  the slave child should be kept together with its parents (no matter mother or father, both) until 18 years old. The separation of slave child from its slave mother or slave father in such a small age (7 year, even 6 months according to Imam-? Malik) in Islam had made the child and the parents very very unhappy for many centuries. This separation process is a violation of human rights.As a muslim I know that Islam gave many many rights to slaves. We learn this from Qur?an and from Hadiths. But I?m really confused about non-muslims critics about separation of slave child from its parents in Islamic Law mentioned above. I have two sweet girls, one 4 months and one 7 years old.  For me it would be really unacceptable to separate from them in such a small age (7 years  or before) if I were a slaveI would be pleased if yould clearify me by considering and answering the 4 items mentioned above and non-muslims critics.Thanks a lot. May Allah be with us.