Have you ever seen children washing their hands when they think their parents are not watching them. Perhaps a quick dip of the hand into the water only. Teach them to wash hands properly and check how they are washing their hands again and again. It is of no use getting angry with them. Every parent knows a command has to be repeated perhaps a thousand times.
Children are only children.
No use of getting angry. Measure lead and other heavy metals in the blood levels of your child, especially if your child is between 6 months to 1 years old according to one source . Follow the links to the Greenpeace investigation above for more advice. Lead levels in blood can be measured rather easily. Use good ventilation in all sleeping rooms if you suspect lead in the wall paint or if you cannot get hold of experts.