Got a food processor? This is probably the appliance that I use the most in my kitchen.
My kids will not touch mushrooms or onions. They do love them, though, and here's how I know this fact. They eat them and ask for seconds. They just don't know that they're eating them.
When I want to add some flavour to a dish (in this case Shepherd's Pie), I whip out my handy dandy petite size food processor. I grind up pieces of onion, mushrooms and peppers until they're a paste. It helps if you add a bit of liquid, like olive oil, to the mix. You can hardly tell what's in the mixture once it's ground up. At this point, if the kids ask me what I'm making, I just say, "I'm making flavour!" to which they reply, "I LOVE flavour!"
Muah ha ha!!
When that's done, you just mix it with your ground beef and cook it.
See? No, you don't, because you can hardly even tell it's in there.
This would work for any ground beef dish - meatballs, meatloaf, spaghetti sauce or even tacos. I've even put fruit & veggie pulp from the juicer into meatloaf with the same results.
Just remember that the secret to this is to call if flavour. How can anyone refuse flavour, right?