Don't let anyone deter them. Inform them of what computer science as a career is really like. Girls may perceive it as a job spent all day in a cubicle with nothing but a machine. When they ask, don't tell. Girls tend to ask for assistance when something won't work.
Boys tend to try to figure it out.
Encourage them to be daring with the machine. It's a real confidence booster when they succeed. Only step in if you really need to, and then try just a hint or help them to read the manual. For young girls, purchase games that appeal to them. The more time a young child can spend on a computer, the more confident she will become with the machine.