I can teach English to my kids.

I wasn't sure what you were trying to say when you were talking about teaching your children English, but I've corrected it in the way that I thought made most sense.

Well done! :) 

I can teach English to my kids.

I can teach English my kids.

(I can teach English to my kids)

("In Japan, most people of my generation can't speak English" may be better, but "In Japan, most people can't speak English - especially people in my generation".  If you use the word "our" it assumes that the person you're talking to is of the same generation as yourself)

Nowadays, mothers know that they should have their child learn English.

("Nowadays, mothers understand that they should have their children learn English." -- in this case, "children" would refer to one or more whereas "child" is singular)

One of my friends bought expensive teaching materials for her kids like my monthly income.

("One of my friends bought expensive teaching materials for her kids - materials that cost as much as my monthly income."  I would probably say "One of my friends spent as much as my monthly income buying teaching materials for her kids" - you would substitute "his" if your friend was a man)

If I had child, I could chat with my kids in English so they could acquire English before they know.

(If I had a child, I would chat with my kids in English so they could learn at an early age)

Although I spent a lot of money to learn English, I can have some benefits.

(Although I spent a lot of money to learn English, it has many benefits)

You are doing very well, Hisako-san.  Even if some of your sentences can be constructed better, your writing is understandable which is the main point of communication.  I'm honored to help.