Our elderly parents house is empty due to them being abroad. It's 5 mins away from all of us. Sister, husband and kids keep going over every now and again to hang out out there or for "a change of scene" and kids play in the garden, as well as WFH there.
But also BIL, his wife and kids are doing exactly the same because their kids are bored and need to get out of the house. Also occasionally WFH there. They both have houses and gardens of their own, even though BIL's is tiny. (Going on different days).
AIBU in thinking that this is a risk to them and the kids? They are still going to supermarkets which still pose a risk so could potentially be contaminating the house and each other's families.
The house is generally secure with a car parked outside, an alarm, a Ring doorbell and great neighbours.