Once you give a warning letter to your domestic helper, the relationship will be broken. Do you really want to do it? You know, your domestic helper is living with your family, your kids are taking care by her and she handles food processing daily. Please think about it first.
If you 100% want to WARN your domestic helper, please PM and I teach you how.
I am also considering issuing a warning letter to my maid as advised by the agency, however having read your message, i wonder there are other better methods to warn her so as to make her improve. Fire her is the last thing to do