I've been looking for this as well. I downloaded multiple messenger apps on a tablet I gave my niece so we could talk, but they all require a phone number.
One app I know you can use without a phone number, is FACEBOOK messenger. However, she is 11 and I'm not making her a FB to chat with me. But if your kids already have FB, because most do, then its simple and free to use the messenger, as long as everyone has a FB. I actually use the messenger now for all my texting, including non-FB texts simply because I got tired of having 2 apps to open. The only thing I don't like is Favorites/Birthdays/Online sections that always reappear.