Aren’t there locks on the windows? Permanently altering the house for thousands and thousands of dollars seems excessive for a brief problem. Worst case screw plexiglass glass over the interior lower portion of the window for now.

Look into installing window guards of the kinds used in apartment buildings. Your local fire department may have ideas as well.

Remove the curtains so the child doesn’t pull them down by hanging on them. The windows should be tough enough a toddler can’t accidentally break them.

If your toddler ran at the window full steam, he'd bounce off and land on his butt. No way he's breaking the glass. Not happening.

Furniture in front of it (not ideal from a design perspective) but consider that a temp fix for a temp ‘problem’ - not sure what your fears are about? Breaking the glass with a plastic toy? They won’t be able to even open a window til much older….

So keep the windows closed, like you would the door to a basement staircase.

I agree with the commenter who said to remove the curtains if you think your toddler will try pulling on them, etc. (That said, we had long blackout curtains in our toddler's room starting at age 2, and it was never an issue but you know your kid best.) You can get pull-down shades instead and then pull them down for bed and pull them up each morning. The window itself shouldn't be a hazard. I actually think a low window is kind of nice for a toddler. They can enjoy the view for once. The kid can have fun making suncatchers to hang against the window or you can put up cling-on decals. When old enough, Crayola makes window markers for kids.

You can add locks to the window sash if it does not have any currently. You can also insert a block of wood that the window cannot be raised without removing the block. For security, I would recommend both of these even if you didn't have a toddler.

Like @ShadyWillowFarm said, you could remove the curtains but speaking as a window treatment professional... if you replace them with something like blinds or shades, please use cordless or motorized products. Young children like to play with cords and can easily get the cord wrapped around a finger or an arm or their neck causing injury.

Your biggest challenge will be if you plan to open the window to let fresh air in. In that case you may need to consider some sort of grate or guard system that mounts to the outside of the house.

Did the last 2 commenters read this thread? Nope.

I’ll speak to the window glass breaking. My toddler grandchild loves to pound on windows. Their house has original 1978 windows and they are single paned and very breakable. For now, they are using a tension gate like barrier with bars. (The windows are inset about 6”). Certainly not pretty, but it is safe. As others have said - no cords, no sheer panels.