When you say they're eating poop, how much do you think they're eating, and are you sure they're actually eating it? It's not uncommon for budgies to clean up their environment by picking off bits of poop from around their cage. Mine do this if they decide I'm a bit too slow to clean their perches etc. Do you think this could be what they're doing?
Kellie is correct.
Budgie's often pick at different things they find in their cages. It doesn't mean they are actually ingesting it.
It is very important budgies' cages be cleaned on a daily basis for the birds' health and well-being.
Most budgies will pick at bits of poop in their cages from time to time so it isn't something to be overly concerned about.