After you have withdrew money from RESP, your children should receive a T-slip showing the money that you withdrew. If you forget to report the amount from the slip, CRA will adjust the return (if they find out).
If CRA adjusted the return, you don't have to do anything. However, if this situation happens frequently, CRA will slam you with penalty.
If CRA has not discovered and adjusted the return, you can download a form call T1 adjustment. Complete it and mail it to the CRA (with your slip)
If you have access to "MyAccount", you can adjust your return online.
Hope it helps.