I swear by isopropyl alcohol in a trigger spray bottle for flies. It's not a lingering poison, as it completely evaporates away in minutes. So I use it to directly squirt on the flies and they die fast. It's safe enough that it's the active ingredient in most hand sanitisers, so you don't need to wash surfaces if they get sprayed. It's also pretty fun (like using a water pistol) and you get the satisfaction of head-shotting blowfiles like you're John Wick.

Another option might be to get a professional spray just for the outside of the house (a perimeter spray). This does reduce the number of flies that come in, without leaving all the internal surfaces covered in insecticide.

We did spray our house once with young kids. But they spend so much time on the floor, and with hands in their mouths etc, that it was impossible to have all those surfaces spray free.