In Japan the Katakana written language has been increasing steadily. I read a book that said that pre world war two and up until the 1960’s Katakana written foreign languages was very low, but from the 60’s it started to steadily increase.
In 2005 when I first arrived in Japan, teaching Halloween and Christmas for example, was easy and the children seemed to pronounce the words perfectly but from around 2010 onward, when Christmas and Halloween became really popular, the ability to learn Christmas and Halloween words with an English accent became very difficult for students. Why do you think this was? Because the now popular Christmas present, Christmas tree, trick or treat started to be advertised more and more in Katakana which of course the kids pick up very fast..
Having more and more Katakana English around and spoken in the children's formative years makes a big difference in their ability to hear different sounds, which as I talked about starts dropping off from 10 months and finally sets in around seven years old.
What makes a difference is starting English very very young, even during pregnancy, and being able to see English words written in English as soon as possible.
Unfortunately according to research and of course depends on the individual, our ability to hear new sounds easily is pretty much finished by three years old. So though they can get a nice accent if they try very hard, learning English after three years old the student will have to make continual effort to refrain from speaking in a Katakana accent. Of course these are all generalisations.
So starting English as soon as possible in my experience and opinion is of the utmost importance for your children’s future.