jefmcg
Guru
Daily capping works with oyster, weekly doesn't - but you won't need that.If it only applies to devices and cards (and I assume by that you mean contactless credit cards), and not oyster cards, I might be out of luck though -unless I consider buying a cheap phone which can be a payment device.
For the adults, just get two oyster cards at Heathrow, put £20 on each and just use them. It's easy to top up if you need to, but you'll probably max out at £6.60 most days.
You can get a refund on the deposit and any remaining credit from any manned tube station when you are leaving.
For the kids, you can order the photo cards online and pick them up from visitors office at Heathrow if you are arriving while it is open. At least that's my understanding from skimming web page.