You have my sympathy, as I suffered from noisy children for years, to the point I even considered moving! Unfortunately, as Jane commented, parents to get immune to it - or they go elsewhere in the house where they don't hear it, and leave the au pair in charge of the kids! I don't think it's unreasonable to ask them to find quieter games which don't involve banging or knocking on the walls - maybe some craft activites?
Despite the current restrictions, as far as I know Wandsworth Common is still open, and if parents are working from home or on furlough, surely they can take the children on the common to let off steam? (actually easier now than when parents were out at work!).
As a footnote, as the weather gets warmer, may I ask everyone please to go easy on BBQs, as being at home we all need to enjoy our gardens without being assaulted by stinking greasy smoke.....!! Maybe cook indoors and picnic outside? Many thanks!