What would be your advice on someone investing for their child? Is there a good long term low risk investment choice?It'd be the same for anyone considering investing: do your homework first. Plunging money into anything you don't understand is a fail waiting to happen. Just ask anyone who got greedy during the housing bubble.
Second, you need to determine what exactly you're investing for: so the kid will have savings later? College? First house downpayment? Remember that different investment choices make sense for different goals, for example 529s & certain IRAs have tax benefits for education savings.
On the other hand depending on the child's age and when you'll want him or her to use the cash, you might want to buy stocks, which can be riskier but provide better returns over the long-term, or bonds or money market funds which have the goal of protecting your principal (the amount you initially invest) and could make sense if you want access to money in the shorter term shorter term.
I have to warn you, though, that I think looking for a long-term/low risk investment might be misguided. You always need to invest to your own sensibilities and risk tolerance and keep in mind the length of time before you'll want to cash out of your investment.
Remember, though, that as a general rule the longer the investment horizon, more risk you can afford. You can stand to have significant equity positions (stocks) in a portfolio if you're not planning on using that money until 10 years from now, because of the likelihood that long-term gains in the market will more than make up for short-term losses. But if you're trying to stash away some cash for something you want to buy next year, putting that money in the market would be foolhardy.