@mikomeko, suggest that you become familiar with THS Guests & Third Party Policy. Related terms seem very clearly stated. Applies broadly to any type of non-housesitter person at the pet parent property.
In two of our housesits, at time of housesit confirmation then we were asked by pet parent if their university age child could spend one night at property during housesit (they were each leaving parent holiday and returning to university). In each case we discussed and consented, and outcome was perfectly pleasant. But in each case, pet parent acknowledged that we were within our THS rights to simply say ‘no’.
There was a recent Forum thread about saying ‘no’ to any requests from pet parent that are unreasonable.
https://support.trustedhousesitters.com/hc/en-gb/articles/360001881117-Guest-third-party-policy