Twenty-five businesses, run by young entrepreneurs from as far away as Southborough, will offer everything from sweet treats to ferret houses and scrap metal garden ornaments. One of the participants, 9-year-old River Gammon-Rainville of the Warwick Community Homeschool Co-op, has been designing and 3-D printing an assortment of items for his business, River’s 3-D Designs. There is an element of competition, too. Businesses will be divided into two age groups — 9 years old and under