This is a novel way to observe wild birds. This feeder works with sash style double-hung windows.
Birds come into the room to feed, while you observe them through a transparent panel.

SAFETY FIRST: This requires you to open your window to accommodate the feeder. There are specially designed sash-locks that will secure your window. OR, consider removing the feeder and closing the window when you're not at home or at night.

SOURCE: This is where I got this feeder, there are many other variations on the market. This is not an affiliate link: https://hibirdie.com/products/window-...

LIGHTING: I added battery powered LED light wands to the sides so the children could see the feeding birds better, and it worked!
This is the light want that I used for this demonstration (affiliate link)
https://amzn.to/3xlbrvr

FEED FOR BLUEBIRDS: I found that by offering black soldier fly larvae, I can get Northern Bluebirds to come to the feeders. This is an affiliate link to the black soldier fly larvae that I use: https://amzn.to/3J51vsz

MOST DIVERSE FEED: You will attract the widest variety of songbirds with Black Oil Sunflower Seeds (affiliate link) https://amzn.to/4cHy4dQ
Save money and get some at Tractor Supply or your local garden store.