Sometimes you have to let them just be kids.....
If that means pizza, then fine, but also include some carrots or fruit (strawberries, grapes, etc.) to balance it out. If you restrict them to ONLY "healthy" foods, they will end up over eating those restricted foods later in life when they have no one to tell them otherwise.
Limit the candy and snacks to small portions, and only once or twice a day (never before lunch). Kids need protein as well as veggies, so let them have the occasional burger or whatever, just balance it with something good.
This is just my opinion. My kids are 8 and 6, so we battle with this daily as well.
God bless,
Rob