To know the side effects of the cream, you need to know the components in it. These creams contain active components that increase the deposition of fat in the applied area. The most effective of them are known to contain herbs and scientifically, side effects are said to be rare.
However, the general side effects might include weight gain since the cream is absorbed by the skin and circulates through other parts of the body. Other effects include hair loss, irregular menstruation, stretchmarks etc and like I said they are reported to be rare.
I know a girl that used such creams. It worked for her in the beginning but when she stopped, she ended up with the body parts looking like a deflated balloon. You need to be sure of what you are buying. Personally, I feel the best way to achieve bigger hips is regular prescribed exercise and diet.