warm water lavage (irrigation) is safe at virtually any age but unless you plan on getting very very wet the child should be old enough to cooperate and sit still while you do it. you need to do it properly and with the right equipment and you need to have a good home otoscope (a home medical device to look in the ear canal. about $30) to check your progress. don't use any instrumentation inside a child's ear unless you have been trained to do so and are experienced.
how young can a child be to perform an ear irrigation?