Well, here goes:

First of all, yes, the parents will eat some of the babies. A lot of them will be able to hide until they get too big to eat, though.

Secondly, you have a potential problem on your hands. Guppies breed easily and frequently. In a twenty gallon tank, you should have no more than twenty one-inch-long fish, at the very most. You will very likely have more than that just from this one breeding. And, as soon as these babies grow a little, they will all breed ... and so on and so on. That is why guppies have become one of the popular "feeder" fish ... they are cheap and easy to breed.

I personally could not raise fish to sell to a pet store, knowing they will be fed to bigger fish. But, lots of people obviously can do it. If you are not going to raise and sell feeder guppies, then you need to do something, ASAP, or you are going to have major problems when your twenty gallon tank is full of hundreds of guppies, and they all die from disease and overcrowding.

Do you have plain or fancy guppies? Plain guppies are easy to sex, the males have colored spots on them, and the females are solid tan in color. You need to separate the males from the females as soon as possible. If you have fancy guppies, I personally don't know how to sex them, but I'm sure a pet store could help you.