Don't push them on pressure, they're just 4-year-old children, they will be improving.
At first I thought your English is not good enough, and I was thinking of you, should be hiring a well-pronounced teacher. But you said you're quite good. Then there's no need to worry about.
That's just children's problem, I bet you might have heard some American/British children say "Dad, I thinkai could do bedder dan dat!"
How cute are they :)
Finally, children - just like all of - imitate the voice which they hear at most. And that will become their accents. So TV is fine, you are good.