Hi ARJD,
1. It would help to give the helper written guidelines on important issues which are non-negotiable such as taking children without your knowledge and permission and leaving home without a working phone, children's food/routine and standard of hygiene you expect to maintain etc etc. You could write it down, explain it to her, ask her to repeat and then put it on the fridge or somewhere where she can refer to it when in doubt.
2. When you send her out with kids or to shopping or ask her to do any work, try and give her very clear instructions, slowly and ask her to repeat what you have said to ensure that you both are on the same page especially if her language capability is not fluent.
3. It is important to remember that most helpers are not trained domestics and your helper may be doing this job for the first time in life and her past experience as a farmer, vegetable vendor, nurse, teacher or a sales person may be very different to working in a home in HK.
4. If you do not see improvement within a week or two of adopting a systematic approach then it is a matter of attitude, work style and understanding in which case you both would be better of in looking at other options especially if you are a working mother with young children.
5. Most helpers with good nature and attitude learn fast and able to perform domestic functions fairly quickly especially if they care for their job and stability. But you can't have much luck with the job gypsies.
Hope this helps.