I put a cockroach killer under the table. Watch out for the baby not to touch it, Some cockroach medicine is in the cabinet beneath bottles. The cockroach killers are in a small plastic containers.
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Are the sentences grammatical? Which expressions do we have to use, 'killer' or 'medicine'?
Yes, "killer" is the right term to use here. You can see it in all the different ads and products available!
http://www.google.com/search?q=roach+killer&oq=roach+killer&aqs=chrome..69i57.2208j0j7&sourceid=chrome&es_sm=119&ie=UTF-8
I'd write it this way, though:
I put roach killer under the table. Watch out that the baby doesn't touch it. Roach poison is in the cabinet under some bottles. The roach killers are in small plastic containers called Roach Motel!
The sentences are gramatically correct.
To clarify, if you are referring to products that are specifically designed to kill cockroaches, you should use the term "cockroach killer." On the other hand, if you are referring to medication or treatment for cockroaches, "cockroach medicine" would be more appropriate. Based on the information you provided, it seems like you have placed cockroach killers (products designed to kill cockroaches) under the table and cockroach medicine (medication or treatment for cockroaches) in the cabinet beneath the bottles.