You said there are many possibilities for bugs.
If it is an ant, it comes from several places: there may be a gap in the external wall of the building itself, and it will climb to your home from the gap.
Or marble is hung on the outer wall of your house, and ants climb along the wall to the platform of doors and windows and then come in through the gap of profiles.
Or there is a floor drain in that part of your house, which never or rarely receives water, and ants climb up along the pipe wall.
Find these three sources, sprinkle cockroach killing powder or ant killing powder or call a professional ant killing company, and make sure that children are not exposed to those toxic substances.
I hope I can help you.