I am 28 years old. i am interested in child psychology. but i am completed B.A in History. can i do my career in child psycholog

Glad to know that you have explored your interest and are interested in doing all thats possible to fulfill your interests.

While you say that you have done BA history,it is difficult to go in for a regular MA in psychology as it requires one to have psychology till the Bachelors.

Nevertheless you can consider courses like:
1)Masters in psychology via distance education (IGNOU,Annamalai)
2)PG courses in child psychology or counseling

Hope this helps.