Child Developed A Rash In Diaper Area, Legs. Small Red Bumps With White Heads. What Could This Be?ch

Hi, My toddler (18 months) has developed a rash in her diaper area. Yesterday I noticed about 5 small red bumps almost like pimples, kind of looked like they were forming white heads also, but they're so small it's hard to tell if they have white heads. When I push down on a bump, she complains that it hurts. I babysit a baby that I noticed have the exact same bumps on her but all over her diapy area and legs. The bumps on my daughter look identical except she doesn't have as many. She hasn't had diaper rash in months, what could this be? Could she have contracted some kind of infection from the other baby? I used the same changing pad with them both.