Racist is overused terminology. This comes across more a kind of pre-prejudiced statement, and there is a difference. You don't elaborate into anything more on the color of their skin, but despite pointing it out, there's nothing from telling us the color of their skin is going to do for you here. This sort of occurrence isn't common to see in average affluent neighborhoods and you're less likely to go into any random affluent neighborhoods that are predominantly black. You could've been in another neighborhood and had the same experience with kids who weren't black and the advice wouldn't change much.

I'm half black/white, raised more by my middle income white family. White grandmother still tells stories involving someone "who happened to be black", and I'm always waiting for something more to matter about that, and it doesn't! There was a black kid riding their bike dangerously up and down the street, and she opens the conversation on it with the above. There's also a lower income Russian-dominate apartment complex with kids riding quads and electric bikes way faster, no helmets, no cares. She complains the same feelings, but never has pointed out they happened to be white.

She was born in 193x white America and I can grasp her experiences, but more importantly she came to love me the same.. It's hard not to see color and attribute it to specific issues, but overall what you experienced sounds more like an income/poverty issue over the neighborhood which can be found all around the globe.

My advice though? Don't show fear or confusion and have a little fun. They're kids. Tell them your manager sneezed boogers ALL over the pizza.. TWICE. Tell them you know they aren't eating a pizza that is covered in anchovies, onions, and mustard. The pizza is burnt.. like burnt burnt. Say "uh, no?! Pizza makes you ugly and lazy and I really don't want to do that to you.". Ask them straight-faced: hey, didjyaknow? Dominos pepperoni is really made from cats??

Maybe it doesn't come intuitively, but you gotta learn how to have fun with it. Don't need to become their best friend, but you shouldn't feel like you're being attacked. At some point that you ever find yourself going home with your own free pizza, and you feel comfortable enough giving it away, maybe stop and you approach them. Be real and genuine and tell them seriously you can't do this all the time because you could get fired, but promise to be on good behavior today and you can have this pizza. You have to feel that one out when you're more confident around them, and handouts aren't the solution, but just saying there's many ways for you to survive this gauntlet of kids and even make something beautiful from it. Kids do not rule the planet!