One way of passing state from parent to child

ContentView.swift

// We are a way for the cosmos to know itself. -- C. Sagan

import SwiftUI

class ChildState: ObservableObject {
    @Published var childData = 0
}

struct Parent: View {
    @State var parentData = 0

    var body: some View {
        ZStack {
            Color.black

            VStack {
                Text("Parent state \(parentData) -- click me")
                    .onTapGesture {
                        parentData += 1
                    }

                Child(parentData: parentData, childState: ChildState())
            }
        }
    }
}

struct Child: View {
    let parentData: Int
    @StateObject var childState: ChildState

    var body: some View{
        Text("Child state \(childState.childData) parentState \(parentData) -- click me")
            .onTapGesture {
                childState.childData += 1
            }
    }
}

struct ContentView: View {
    var body: some View {
        Parent()
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}