Did my dad give my kids COVID?

I’m super confused right now and frustrated as all three of my children have COVID (4,3,and 4 months) my husband and I are fully vaccinated as are my parents and brother. Well last Saturday my parents came to visit for a week and by Sunday my dad and brother had a horrible cough and spent most the week sleeping(though they kept coughing and coming near my girls even though I asked them not to) Well on Wednesday my girls started to show signs of the same thing my brother and dad had, so once my family left I had my girls tested and they are all positive. So I let my mom know so they could get tested as well and isolate. Well this morning they got tested and said they were negative… my girls haven’t gone out anywhere since before my parents visited so I know they didn’t get it elsewhere, and they had the exact same symptoms as my dad and brother. So why would they test negative and my girls positive?