How should I explain to my own child that there's a child with a disability in their class?

Without treating it as something special, convey the child's abilities and characteristics concretely and in non-negative words—"there are all kinds of children in class; ○○ is a slow talker, or loves to move." Talk on the premise of "a member of the class," not "poor thing." Since the diagnosis name is the child's and family's information, take care not to spread it on your own.

For a child, you could say: "There are all kinds of children in class. ○○ is good at ○○ and slower at ○○. If they look stuck, try asking."