There is also weekly capping, Monday->Sunday but only applies to devices and cards, not to actual Oyster cards.
Note to the OP, you will need a separate contactless card for each traveller, so unless your kids have their own cards or you have spare ones with a different number that you are happy for them to carry, you'll need oyster cards for them
(and if you pre-order a photo-oyster card for them and pick it up at heathrow, I gather their travel is free. Who knew? --- yep, all the other posters on this thread knew

)