I'm no poet by any stretch, but since Halloween is tomorrow and nobody else has replied, here's one for you. It may deter some kids from dumping the whole bowl in their bag if they think they are being watched??? I've seen that done before! Maybe you can tweek this one to work.

Because you are so sweet,
I've left this special treat.
But if you don't want a nasty trick
1 or 2 candies ONLY you must pick!

Please do not laugh, I thought I would give it a try....

It's, Halloween, the night of magic
But eating too much candy might be tragic
Pick ONLY 2 pieces of candy tonight, to be sweet
So the next Ghost or Goblin can also enjoy the treats !!!!

Here's a short but sweet one that I'm going to use:

Please take one bag,
but not a few
So the next ghoul or goblin
can have a treat, too.

Also, I have pre-made bags, which is why I worded it that way. I suppose you can change it if need be.

Please take a treat (or two or three)

But the zombies are watching and they will see

If you take the whole basket unkindly.