The two have nothing to do with each other.
The Oyster Card is used as a means of prepaid automatic payment of much transportation within the 6 main transit zones of London. Heathrow is at the end of zone 6, Marble Arch is in zone 1.
Friends and Family is a discount card providing a 33% discount on adult tickets and 60% off child tickets on National Rail so might be helpful on the trips to and from Cambridge and then out to Stansted. Or are you not returning to London and are you going straight to Stansted from Cambridge?
Are there 2 adults or just you? The Friends and Family card costs €30 so you have to get discounts on €100 of train tickets before you even break even.
If you wish to avoid the use of an Oyster Card you can get a paper Travelcard (possibly not available at Heathrow) for zones 1-6 at 1/3 off using the Friends and Family card, but as I say the logistics could be difficult at Heathrow. Heathrow has no National Rail sales facility and the Heathrow Express and Heathrow Connect are NOT National Rail facilities.
You can't get any Friends and Family discount on other zoned Travelcards - particularly not on zone 1-4 and zone 1-2 cards which would likely be what you would need while in London.
Child tickets on British trains are pretty cheap already. Children under 5 travel free, and 5 to 15 are half fare. So 60% off half price is pretty cheap.
BUT - you have to work out if you will have well over £100 of rail tickets so that the card makes sense.
You can't pass an Oyster card around. Each card is for one person.