Hi Joanne,
Can you tell me how old the child is please?
As far as the adults are concerned you might be better off using a contactless bank card. You’ll cap at the zone 1-2 daily rate of £7.70, but if you don’t actually make enough journeys it’ll only charge for what you do. You also avoid the need to spend £5 on the Oyster card which you’d need to keep for a year to be able to get that back.