My grandmother's house has something similar, but not quite as deep. Toads kept jumping in and couldn't get out. So I leaned some timbers in there for them to climb out. Do you know of a better solution for something like that? The timbers look kind of trashy...
There are many types of covers available that can help keep the little critters out. Whatever solution you choose you'll want to insure that the window is not being blocked in case of an emergency. I usually don't recommend covers because they get covered in snow and children will not be able to lift them off. Best of luck solving you toad problem.