If you're doing your best to make sure he is in their lives then send the kids to visit. Otherwise you are doing your best to keep him out of their lives. Simple.
Of course you're getting the occasional abusive text. Dad wants to see his kids and you're stopping him.
I disagree with Louise. OP has lots of excuses for not sending the kids. But none of them relate directley to the well-being of the kids. No established child abuse / neglect? not accusation, not opinion not excuses. HARD EVIDENCE police reports, Doc's interventions, impartial third party documentation relating to kids...
Ok - so kids are 12 and 13? My situation is not entirely dis-simmilar. Although I have court orders and my kids visit the ex 8 times a yar. But if the ex witholds them even with court orders I'm still having to apply to court to have them returned. So that is the whole not sending them because without court orders arguement blown out of the water... (kinda - keep reading)
So my ex lives in crazy land. Ex thinks that now that the eldest is 12 the child can choose to live with the ex. Ex applies pressure on 12 yr old, even going so far as to show the child the wonderful schools that the child could choose to attend. 12 yr old has expressed no desire to live with the ex and feels stressed about the pressure. I have communicated this to the ex but the ex doesn't want to hear it... So in many respects my situation is worse than your's because I have consent orders. I am compelled to send the kids and every time I worry that the eldest in particular wont be returned because the ex will claim the kid wants to stay...
So what to do with this concern about the ex keeping the eldest and claiming that the 12 yr old wants to live with the ex? the answer? 40 cents. Little bit old school, but go with me. 40 cents that is the cost of a phone call from a phone box. They still exist. But my point is this... It is even easier these days. The kid doesn't even need 40 cents and a phone box. My 12 yr old knows how to contact me via a range of telecomunications / social media. The kids know about stranger danger and because 12 yr old has told me she is worried about the ex not returning her - I have had conversations about what to do. Those conversations are not dis-similar to the stranger danger message... So unless the ex locks the kid in the basement my kid can sort this one and if the kid is locked in the basement the cops will come a knocking....
Desperate for the kids to see dad? right? Your doing your " best to make sure he is in their lives despite the circumstances." I disagree. If you're that keen then apply to court for court orders that say when the kids spend time with you and when they spend time with him. ... Heck, while you're their you can request the court orders drug tests... Once you've done that, you can truly say you're doing your best to make sure he is in their lives.
Ex and partner have been to jail. Ok, but not for child abuse... And don't we live in a society where once you've paid your debt to society and released you have done your time?
So let me play a game called what if... What if he applied to court for access. Would he get it? Yup I reckon he would. So sure you could stop the kids until he does that court application, but that is the opposite of the wole I'm doing all I can to facilitate access argument.
Final rant... Drug addicts don't want kids they want drugs. If he has got his life together enough to contact you for access, I'm willing to bet he has gotten his sh*t together enough to look after them for a week or two.