There's also Trader Joe's, though that might stretch your budget even though their prices are very good.
I'd say Costco is a very good bet. Depends a bit on what your serving resources are. Do you work out of a place with a kitchen like a school or church? Does everything have to be completely ready to eat?
Still, Costco has a variety of decent big pizzas you can heat up on site.
Very cost-effective cold cuts so that you can make almost any kind of simple sandwich a kid would want.
Bulk fruit. Nice platters of pre-cut mixed fruit.
Blocks of kid-friendly mild cheeses you can cube.
Party-size platters of hummos and variant dips for raw veggies.
Huge bakery area for cookies and other sticky sweets kids like.
Juice boxes, chips, salsa. Ready made guacamole in packs.
Huge aisle for all sorts of snacks from traditional (chips, pretzels) to rice crackers, trail mix, nuts, etc.
I really think you'll have the best value and widest choice options at Costco.
That would
"Strange how potent cheap music is."