I got tickets for my 9-year-old and myself. I wanted to add my 11 year old niece but could not find a seat for her next to us. Is it safe for both (9 and 11 years old) to seat together and I can seat far away ?

I would say yes...but not too far away...as long as you can see them

It really depends on the show you are going to see and the crowd that follows...Generally speaking its never a good idea to leave your children alone while you sit "far away"...

Let the ushers know your issue. They may be able to help with seating especially if its for Anastasia. I attended opening night and there were lots of youngsters. If seat exchange falls thru the ushers can be on alert to help you keep tabs of your kids. Good luck!