All good suggestions so far, tho if you have flies etc., in full, fast flight, they're probably in before they get a sniff of the citrus or the sugary drink in the cup on the windowsill.
What about a hanging bead, chain or plastic strip curtain? String it up across the doorway to create a visible 'barrier' that the insects will not want to fly into but it won't stop people passing through.
Only drawback is that smaller kids absolutely love to play with these and can easily pull it to pieces - stern warnings about not playing with it would probably be needed.
In Spain, I've visited houses where they have the pull down mesh screens for windows and they are a fantastic job for keeping out the mozzies. If you wanted a long term solution, you might invest in a light wooden or plastic screen door with similar mesh fitted that automatically closes after being pushed open to let people out - the children would quickly learn how to use it.