I, actually always have this problem => 生非滋 :(
Believe me, I am really quite an expert in this diseases!!
Do you know the reason behind?
非滋, formally called 口腔炎, is a symptom of insufficient Vitamin B2.
If the patient is adult, I will then suggest to take Vitamin B2 pill. Simpliest way!
( * Remember, not B什 but only B2 particuarly!!)
However, as this time is only a 10-year-child,
I would suggest to take more Vitamin B2 food in usual daily life!
e.g. all kind of meat (beef, pork, chicken, lamb, fish), egg, milk, cheese, yoghurt,
oyster, peanut, bean, mushroom
Is your child like vegetables more than meat?
If that is the case, you should teach him/her to eat more meat then :-D
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BTW, the above is for prevention.
As long as your child already have it.
Besides applying medicine, I would like to suggest
using 鹽水(1/4 tsp salt for one cup water) 潄口刷牙!!
Just like usualy, 2 times a day will be fine enough.
As it's only 1/4 tsp salt for one cup water,
that cup of 鹽水 should not taste different actually.
After using this method to 刷牙,
非滋 should be recovered within 3 or 4 days.
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As it's only a 10-year-child, I really don't want to suggest to use medicine.
Anyway, I personally think 日本 [白兔牌] 的 [吻百蜜] is quite good.
Taste just like honey :-)
However, again ...
the best best method should be "Eating Habit Review" absolutely!!