Which ear infection viruses or bacteria will surely cause ear infections in a child's playmates?

Neither: There is no statistical basis proving that bacteria or viruses absolutely trigger more ear infections in contacts. Having said that, ear infections are generally considered bacterial in origin, either primary or secondary. That is why antibiotis are often used.

Not contagious: Ear infections are not contagious. Oftentimes, a baby or toddler has a an accompanying cold when having an ear infection. The cold is contagious to another child and that adds a risk of developing an ear infection.