I'd submit that telling him who he can text comes off as control. A boundary is deciding for yourself that you don't want to be around that behavior and then not inviting him.
I've found it helpful to think of H as my ex and set my boundaries where I would have a right to set them with an ex. I can't tell him who he can text. I can't tell how to relate to his kids (except for situations where the law would care). If he is being disrespectful, I create happy spaces for me and my kids and he's not invited. If I already invited him, I bear it and make a different choice next time. I act as if I'm going on with my life. This has made him much kinder to me and the kids. When he saw that I was going to let him go it freaked him out and he started connections with the kids (mind reading here) because he didn't want to lose them too. By making a big deal out of the texting, you are showing him that you're right where he wants you.
I know it's tough to see S7 cry. That same issue is what drove me to the breaking point where I asked H to leave, but you have influence there. You can act as if with your children too. You can tell them that you love daddy enough to let him go if that's what he wants. You can tell them that you guys will be just fine and then GAL with them. It they're sad, just say I know this is hard on you, and give them a hug and a peaceful place to grieve.
Hugs,
FP
M43 H43 M14 T22 when it all fell apart D12 S10 "Never have been happy" 3/2013 EA/PA since 2/2013 H moved out 11/2013 H looking to buy a house where OW can live with him 5/2015 Very cordial, nothing filed