There are a number of people that go no contact w/children. Your children are old enough to be in contact w/their father directly. If they don't want to be bothered w/him, you certainly can't control how they feel or force them to respond to his messages. I, personally, wouldn't worry about it at this point. Why? Because I know you well enough to know that you'll be responding back to his texts again very soon. You'll back paddle and try to explain your stance to him and you don't need to do that. he should already know that if something where to happen to the girls, you would contact him.

If he texts back, let it go. You've said enough and if he's got a problem with no contact, he can talk to his lawyer and explain why you are doing it. Quite frankly, I think he's trying to soften you up to hit you were it hurts, i.e., the wallet.

Sit quietly, the answers will reveal themselves when you least expect them to.
The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.