MThere are 2 types of risk for money market fund. 1st, investment risk.It is a mutual fund--there is risk for any iinvestment. However, since it used for the borrowing between banks, buying bond from companyies and government, the risk is much less than mutual funds of stocks. To reduce the risk on investment, you should pay attention to the investment vehicles of the fund. Generally speaking, higher returns means higher risk. So if you really worry about this risk, choose a local bank, instead of a broker.
Another risk is that the bank with your money fails. FDIC insures up to 100K for each account, so no need to worry the bank with your money failure. This risk is about the same as any other bank accounts.