I am planning to go to SanYaya, on Hainan Island, with my family for a vacation at the end of this month.

The spelling in English seems to be one word. It helps your readers who are not familiar with your country's geography to mention the location. In this case, it's important because you mention "warm water" in your final sentence, but you didn't say that you were going to a beach resort or an island.

I fear the vacation would affect her preparation if she goknew about it.

There is an idiom you could use with "got."
It's "to get wind of" something, which is used about someone finding out something you don't want them to know.
"I fear the vacation would affect her preparation if she got wind of it."

IWe plan to stay there for six days including travel time.

The temperature of SanYya is much higher than Beijing.

To sound less like a weather forecast you could say, "Sanya is much warmer than Beijing at this time of year."

The daytime temperature will stay above 22 degrees Celsius in SanYya at the end of this month, but it will stay under 0 degrees Celsius in Beijing.

We pluralize units of measurement (degrees, kilometers, minutes, etc.)

So we will have an experience of going from freezing cold to warm water[ ].

This comparison is a little out of balance because you won't be in the water the entire vacation.
(When you compared the temperature in both places, that was a balanced comparison.)

Maybe something like this would work:
So we will go from wearing coats in the freezing cold to wearing bathing suits in warm ocean water.

I bought airplane tickets last week and booked a hotel yesterday at noon.

In English, "noon" refers to a specific time (12:00), so it needs "at" before it. It's the same as "I left at 3:00" or "I woke up at 6:00".
"Hotel" is singular and countable, so it needs an article.

The daytime temperature will stay above 22 degrees Celsius in SanYa at the end of this month, but it will stay under 0 degrees Celsius in Beijing.

"Degrees" as in temperature is treated like a noun, so it must be plural in this case.

I haven't told my daughter yet, because she is preparing for her final exams.

I fear the vacation would affect her preparation if she got itknows.

I bought airplane tickets last week and booked the hotel yesterday noon.

I bought airplane tickets last week and booked a hotel yesterday at noon.