Is there a lock for a fish tank top?

I woke up a few minutes ago, and my 2 year old daughter was in my room standing on my dresser over the fish tank. She'd knocked the top off, and she was leaning over it. Its a BIG tank. If she fell in, she could have drown. Is there a lock that I can get for it? I don't want to lock my bedroom, because I want her to be able to come in and get me if she needs something.
 
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