"Small change" can refer to coins of low value, or to paper money of low value, or metaphorically to anything of low relative worth; a couple of $50 bills is "small change" to someone looking for a $200,000 payment, for example.
"Loose change" specifically refers to all of the coins that you happen to have in your pockets or purse that are not rolled or bundled up; if you got two quarters back from a purchase and just dropped them into your front pocket, for example, that's loose change. If you put the quarters into a sleeve for making a "roll of quarters", then they aren't loose anymore. (In a coin purse or a wallet pouch, they're probably still able to rattle about and make noise, which is a key element of being considered "loose", I think.)