ETA required to change planes at Heathrow?

The website says ETA not required unless going though UK Passport Control at Heathrow. I hope this is not a stupid question, but how do you know if you will be going through UK Passport Control when you change planes? We are going from Los Angeles to Heathrow to Copenhagen. I have ETA, but the grandkids don't, so don't want any unnecessary hassles traveling with kids.