Thank you for your query,
There will be no problem of entering of water in to a normal ear. Infection may be aggravated in ears with tympanic membrane (ear drum) perforation.
When child lower that ear, the water will drain out side, at that time you can clean his ear with a soft clothe without entering into ear canal
You can prevent water entering into ears by keeping cotton just in the ear, but not inside the ear.
And immediately you should remove it and clean the ears after bathing.
Hope I have answered your query.