Do guppies eat their fry

Simple answer; yes.

After giving birth the male &/or female will their fry.

You should keep fry in a breeding net after birth.

And keep female in another breeding net or aquarium after giving birth,

this is so that she can recover fully before being mixed with male again.

P.S.

Stress from giving birth & male guppy chasing after her can cause female to die.