What would be the best way to install plexiglass over existing windows?

"Plexiglas" (the trade name product) is pretty good for the task, but mightily expensive.

I believe Lexan is approved for use on doors and windows for safety reasons, and much more affordable.

Attachment depends on what the existing framework around the windows is, but almost certainly silicone caulk won't do the job. It's more of a sealer than an adhesive.

Whenever I use Lexan (or any plastic sheeting) I cut it to fit (slightly smaller than the framing to allow for 'fudge factor') and secure 3/8" thick (by 2" or so wide) lath strips around the perimeter of the sheet; pre-drilled through both the lath and the sheet.

If you don't pre-drill, the screws will crack the sheet when tightening them, and the lath distributes the force of the screws over a wider area, reducing the chance of cracking.

For added weather sealing, you can caulk after the attachment job is finished.

Plexiglass Windows

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RE:

What would be the best way to install plexiglass over existing windows?

I need to cover all the glass windows in my house to keep my daughter from hitting them and breaking any more of them, I'd like to just put it up over the windows, would I use silicone caulking? Please help if you have any ideas, and before I get wise cracks, my daughter is special needs and...

I made some from 1 x 2 wood . first paint the wood , then make frame, Measure window so frame will fit. If window is large put strip of wood divider inside frame. (Otherwise shrink wrap will bow the frame, when it is shrunk with heat gun or hair dryer). Cover with window shrink wrap.

You can then apply 1/8 x 3/16 weatherstrip foam to side that will touch window frame. One of 2 long thin screws will hold in place.

This is about cheapest way, and is easily repaired. I made some for the inside the house, and for the outside. Save on heating bill too.

Plexi glass is very easy to drill through. Is your window trim configured in a manner where you could use some screws? They'd be easily removable for cleaning or whatever. You could even do two halves, upper and lower. That way you could take the top off at times for fresh air. Show your daughter my picture icon and tell her Daffy says Hi.

no need to drill the plexi.

Use 1'' x 1'' square battons , screwed to the frame to make a sub-frame, them install the plexi against it and hold it in place with more battons either nailed or screwed to the frame. Simple !

Remember though if you need to get out in the event of a fire. How would you do it?