Your nephew and niece are taller and bigger (than they were before). We use adjectives such as tall or short to describe a person's height.

We don't use high to describe the height of people. You have several options: Your niece and nephew (we tend to say it in this order) have gotten big/tall. Your niece and nephew have grown a lot. If you use "grown up" instead, it has the implication of emotional maturity, not just physical growth.

Your nephew and niece have grown up a lot.