If you're asking what an Oyster card is, it's time to get a guidebook (or two). Rick's London book is an excellent one for a first-timer; he has lots of logistical details about getting around, what to see, suggested walks, etc. He will also have answers to questions you didn't even think to ask.
Your kids should also be reading guidebooks, looking at videos, etc. And while things may not look the same as on Instagram, it's also a way people learn about things, so they should certainly check it out. There's undoubtedly spots in London that are, currently, the Instagram must-haves for teens, and yours will probably want them too.
Rick's videos are here (scroll down to Great Britain): https://www.ricksteves.com/watch-read-listen/video/tv-show
An Oyster card is how you will pay for transportation. Each person needs their own; you tap it on entry and exit when using the Underground (Tube), and tap on entry only on buses. It costs £5, and you then load it with credit. It keeps track, and never charges you more than a certain maximum amount per day (based on which zones you travel in). For three days, put £15-20 on each card. If you run low, you can "top up" at any station. When you leave, you can save it for a future trip - yours or someone else's. Or, you get a refund of the £5 card cost plus remaining value on the card before you leave - provided you have used the same method to buy it and load it every time (cash or the same credit card).
There are helpers in the stations who can assist you with any questions (including the Tube station at Heathrow airport, if you're taking the Tube into London upon arrival).