Discount Oyster Card for Teens

Yes, the discounted card works from Heathrow on the Elizabeth line.

The process though is a bit convoluted, you can search on here, or someone will chime in, but you cannot do everything at a machine, you must go to a person to get the discount applied to a card. I believe you can go to a machine, purchase and load the card, then seek out someone to apply the Youth discount.

Edit to add, regardless of which terminal you arrive at, you can ride the Piccadilly line for free between terminals to get to a manned counter, and to the Elizabeth line. Second edit, to get on the tube at Heathrow to get between terminals, you do need to get a free transfer ticket (or use a contactless credit card).

Ask any member of staff to add the Young Visitor Discount to each Oyster Card using the staff functions on a ticket machine at the T5 TUBE station, not the Elizabeth Line station.
Yes the YVD is valid on the Elizabeth Line.
The discount is good for 14 days.

If your group is 10 or more, so in your case 2 Adults and 8 Teens, or 1 Adult and 9 Teens then a 1 day Group Travelcard is probably the best option on arrival day at least. It can be purchased from any ticket machine, and is priced (currently) at £10.50 per adult and £5.20 per child under 16 for unlimited travel after 0930 M-Fr, anytime SaSu in Zones 1 to 6 (which includes Heathrow).

This is a paper ticket and the group must travel together at all times.

Thank you for the info - I will look more into a single day pass for arrival day, however we often split into two groups with the other chaperone, so may not work every day.

Game plan on arrival will be to go to T5 Tube station to top off cards and then add YVD with a staff member. Unless the Piccadilly line is leaving soon after, we will plan on then heading to Elizabeth line. (We are most likely staying at a Hotel just off Tottenham Court rd Station, but its not a far walk from several Piccadilly line stops either)

Unless the Piccadilly line is leaving soon after

every 10 minutes, at 6 minutes past every 10 - so xx:06, 16, 26, etc., most of the day... Pretty much all the time.

Elizabeth Line, 2 trains an hour from T5, at xx:20 and xx:50.

Interesting about Piccadilly line. Last June had to wait 30 minutes for it last Summer!!!!

what time of the day did you have 30 minute headways on the Piccadilly Line?

There must have been some reason - the same trains run through central London and if they were on 30 minute headways the stations would be overflowing. Engineering works, maybe an incident?

That is why being fleet of foot and being willing to switch transport sometimes works.
If there is an incident trains will often be turned short of their destination, to try to recover timetable.
That happened last weekend on the Jubilee line where some trains were turned mid line at London Bridge.
As I write the Piccadilly at the other end of the line is having serious issues at Cockfosters with a points failure.
Trains are being turned short of destination to try to give the rest of the line a near normal service.
If that incident was at the Heathrow end of the line they would be turning trains at Acton Town or Hounslow as examples, leading to decreased frequency at Heathrow.
So check the Elizabeth Line instead.
Sometimes even hopping on a bus to another tube station can be the best answer.
If things were really up the creek with both Pic and Elizabeth the National Express bus to Victoria is a back pocket answer. There are at least 4 an hour from Terminal 2/3. Yes they are not covered by TfL fares, but at around £6 each normally they can be the best answer to a problem, to just get moving.
That is what credit cards are for.