const initialState = { termsAccepted: false, showTerms: false };

function reducer(state, action) {
  switch (action.type) {
    case "toggleShowTerms":
      return { ...state, showTerms: !state.showTerms };
    case "toggleTermsAccepted":
      return { ...state, termsAccepted: !state.termsAccepted };
    default:
      throw new Error();
  }
}

const [state, dispatch] = useReducer(reducer, initialState);

...
 const openTerms = event => {
    event.preventDefault();
    dispatch({ type: "toggleShowTerms" });
  };

  const exitTerms = accepted => {
    dispatch({ type: "toggleShowTerms" });
    dispatch({ type: "toggleTermsAccepted" });
    setTimeout(() => handleTermsAndSubmit(), 0);
  };

const handleTermsAndSubmit = () => {
// state is currently: {showTerms: true, termsAccepted: false}
// I want the exact opposite
}

return (
...
Form onSubmit={openTerms}>
...
<child show={state.showTerms} exit={exitTerms} />
)

const [show, setShow] = useState(props.show);
  const [checked, setChecked] = useState(false);

  useEffect(() => {
    setShow(props.show);
  }, [props.show]);

  const toggleChecked = () => {
    setChecked(!checked);
  };

 return (
...
<Button onClick={() => { props.exit(checked);}}>Submit<Button/>
)