Its my job to make sure that the third party looks after them properly and not just ignore the situation and lock myself in my rooom
Sorry, that’s bollocks. You lock yourself in a room because you are at work. It’s no different to being in an office except that the office is connected to your house.
If I had to go into the office, would you still expect the MiL to leave the older kid unattended? Of course not.
Whoever provides the childcare, they are taking responsibility for the kids, that’s the whole frigging point. Otherwise why bother? If you leave your kids at nursery whilst you go to work, are you still responsible? Are you expected to keep popping back from work every 10 mins to check they are ok? Of course not!