I would use plexiglass.
You can cut glass easily only if you know what you are doing and get a perfectly straight edge with the tool. And once cut, unless you can grind down the edge, it will be razor sharp. Should a kid mess with it, or it breaks, you have large shards, all extremely sharp. Plexi won't be nearly as sharp, will flex much more without breaking and can be sanded down on the edges with cheap sandpaper and a block or even rounded with a good knife.
If you really want glass, let a glass shop do all that for you so you know it's done right. Or have them sell you two pieces of tempered which won't have sharp edges and will flex more, though tempered is more sensitive to edge impact and once you have it, it can't be recut.
Plexiglass sheets are cheap at Home Depot. Buy a sheet, cut it to size, then cut the front section off to make a flip up lid or a lid you can slide back. Light weight, durable and safe.
