Do you have to have a DNA test to lagetimize a child?

That depends on the law where you live. In most places, signing the birth certificate or your name otherwise appearing on it as the father is enough to legitimize a child. In other states, that isn't enough and the father has to sign papers and file them in court to legitimize their child and/or undergo a court ordered paternity test. Providing your state (or country, if outside the USA) would help us to provide you with an accurate answer.