Wanna REALLY freak your dad out? Show him a yellow chick & tell him you hatched it out of one of the eggs he left behind at your house.
It's great that you're helping, but I agree that they should either take their eggs with them when they leave or at least not buy new ones every time they come. They're doing it to insult you (in case you hadn't figured that part out yet... you know in your heart there's no need for them to bring a fresh dozen store bought eggs *every* time they come to your house... it's obviously a direct but silent insult to your preference for home-grown eggs). It's gotta bug you that they undermine you that way to your children, too, though thank Goodness your children, as you said, don't really listen to them. That doesn't mean your parents aren't doing it, and you have to ask yourself if this situation is a positive presence in your family dynamic. With direct (not even indirect) insults like that and the undermining of your choices to your offspring, if it were me, I would rethink letting them stay. It's about principles, not eggs.
But I'm kinda a snot that way anyhow, especially when it comes to my family... they'll stab you in the heart and smile in your face while they twist the knife, just like what your folks are doing. I stay away from my family for the most part because I don't think families should treat one another that way. You are respecting your parents' choice to eat crap store-bought eggs, but they're not returning that same respect. You may end up having a difficult relationship with them in the long run, so if you have a good relationship with them now, you should talk to them and explain how you feel and just politely ask them to take their eggs with them when they leave. Or let them know you'll be giving them to a soup kitchen or throwing them out (or hatching them LOL) and that you're not going to feed them to your family and ask them to not be offended by it. Even though they don't think twice about offending you or undermining you to your children, staying open and talking to them about it so that offense doesn't keep self-perpetuating in the family is the best thing, I would think.