I have just had a baby and have a dark line on my lower abdomen

What you have is called the linea nigra and it is a completely harmless skin condition. Linea nigra is a dark vertical skin line that appears in most pregnancies. The line usually runs below the belly button and ends just above the pubis.

The line is colored due to pigmentation caused by the increased estrogens in the body. This is also the same reason why your nipples are darkened during pregnancy. Why this line develops is not known but is most common in women with dark complexion. The linea nigra appears around the 2^nd trimester and fades away after birth. In some cases, a faint line may persist for a long time. Sun exposure usually causes it to reappear.

As far as treatment is concerned, you do not need any. Even though cosmetic surgeons recommend all sorts of lasers and bleaches, the safest and most effective treatment is to leave it alone. The line is harmless and spontaneously fades in most women. 

If you want to get rid of the linea nigra and have money, the best advice is go and spend it on a vacation rather than a plastic surgeon. If you do not have money and would like to get rid of the linea nigra, do not wear a bikini.