The general rule with attendant-wear is that attendants pay for their own garments, including shoes but not including flowers. When you have child attendants, their outfits are paid for by their parents.
There is nothing wrong with the bride and groom deciding to pick up the bill for their attendants' clothing. It's a very generous gesture! But it's not required by any rule of etiquette.
The result of the "attendants buy their own attire" rule is that, if a couple and their child are all in your wedding (for instance, maid of honor, groomsman, and flower girl), they may face rather steep expenses for the honor of attending you. There are several ways that a considerate bride and groom can make the financial burden less heavy without sacrificing an elegant wedding.