Taking the tube Picadilly line from Heathrow to Green park

NOTE: up to four children under 11 can travel free with a fare-paying adult so just you and your husband need to pay. You go through the wide gate at either the left or right side where there is an attendant. You scan in and out and the child scoots through.

If you want to maximize your time, take the Elizabeth line to Paddington. Then use the underground. Tapping with your credit card. Once you hit the daily limit, every other trip is free.

My understanding is that contactless is using your credit card or credit card enabled smartphone. You and your husband need to each have one. Hold your child's hand through the stations as it will be crowded.

. I have an oyster card that I use on every trip. I wouldn't bother in your case.

Edited to remove Heathrow Express as I always forget about the Elizabeth line!

Just take the Piccadilly line; it is direct and will be the fastest and cheapest method to get to your destination. A contactless card is a credit card or debit card that uses RFID (you can tap to pay). You can check to see if your cards have the symbol on this wiki page.
https://en.wikipedia.org/wiki/Contactless_payment

The 8 year old rides free. The rest of you each need your own form of payment. You can also use apple or google pay on your phone.

The Heathrow Express (HEX) won’t save you time. You are going to Green Park so if you took the HEX you would have to change first at Paddington to the Bakerloo line and then again at Baker Street to the Jubilee line. When you add up the waiting for the HEX at Heathrow (the HEX comes less frequently than the tube) and the walking to transfer to the tube and the tube ride with a transfer, this will take longer, cost more, and be more complex. With the tube, you just get on at Heathrow and ride to Green Park.

Thank you so much.

Laura, would you know if each of us need one card? Or can I pay for my husband, my older kid and me using the same card?

You will need 2. Any mix of credit cards or smartphones. One for you and one for your husband. Whichever one you choose use the same method to tap in and out. Keep using this method throughout the day as you ride the bus or tube. If you hit the daily cap then rides are free for the rest of the day.

Edit. Oops missed that the 11 year old will need to pay too.

One form of contactless payment per person. You could use two that ultimately charge to a single account, such a card and a phone.

You need each need different cards. I'd get a one day travel card for the 11 year old.

Why Green Park? Is there something specific your are hoping to do with your time?

Depending on what you want to see during your time, either Piccadilly Line or Elizabeth line will get you there. I have done the trip in with a child and it sucks up a lot of time to take the Piccadilly. Depends on how your kids are with long days and sitting still for and hour each way. Anything under £50 difference in cost is negligible. How much is your time worth? Maybe get to town quickly, see what you can and then take a leisurely trip back on the Piccadilly line. Or don't get fixated with one solution. Stuff happens and there may be a shut down.

One suggestion: if the weather is good, the Diana playground in Kensington Gardens would be fun if they are active kids. I found that when my daughter was tired of being dragged around, she enjoyed going to playgrounds in Paris and got energized.

As noted above, you each need a separate form of payment. For example, if you and your husband each have a card, you can use them even if they are the same account. If you have a payment method on your phone like apple pay, someone can use that.

Obviously the 11 year old won’t have a credit card. If you don’t have an extra for the 11 year old to use, you can buy a paper ”return” ticket (”return” is what we would call ”round trip” in American English) or day travelcard. Info on these is here: https://tfl.gov.uk/fares/how-to-pay-and-where-to-buy-tickets-and-oyster/single-and-return-tickets#on-this-page-1. You can buy these at the tube station in Heathrow,

When you enter the underground, you tap your card. When you exit, you also tap your card at the barrier. The system figures out the cost. This is why each person needs there own method of payment.

The 8 year old doesn’t need a ticket. When you enter or exit the tube, look for the wider, accessible gate (will have a wheelchair / stroller symbol). These gates don’t close as quickly. You tap your card and when the gate opens, both you and your child walk through.

I used Green Park a lot and found it well situated. FWIW, all those London walking tours you see on Youtube seem to start just outside the station, I recognized one of the tour guides. Easy walk down to Buck Pal and then onwards to the War Rooms if desired.