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.
Are you booked on the same ticket? If so, it's an airside transfer. If it's two separate tickets then you pass passport control and your grandkids will need an ETA.
As Simon noted, if this is on one ticket then you just follow the Flight Connection signs for your connection. You shouldn't have to clear UK Passport Control but you will have to go through airport security.
If it is on two separate tickets, and you have to collect luggage, you will have to clear the UK border and the kids will need ETA's.
And technically, with no bags, but two tickets on different airlines, you don't need the ETA (You can still do an airside transfer if you have a boarding pass), but your first airline thinks you are going to the UK, so it will check, and may deny boarding without an ETA. Your ability to argue otherwise with the airline may vary.
Thanks so much for the responses - I knew the Rick Steves community would help me out.