Argh, hard choice.
I said 'enough is enough' to XP wrt our two children.
He never bothered with them and would promise to see the, I'd tell them and DD would get all buoyed up (ds doesn't understand who he is), then he'd cancel with some bollocks about work or w/e.
One time I had driven half way to his house (40 miles away) and got a text saying he was working away, I knew it would be bollocks so we acted like I didn't get it, and I turned up at his mums with the kids (he was living there at the time) he was in the pub
Anyway, I gave him chance after chance, he isn't allowed to call my house or my phone, all contact had to go through his mum, but he still fucked around, now I won't allow them in his sole care, he sees them through his mum or sister.
Could you do that? Is there anyone that would take responsibility for them to see that side of their family? That way you know they wouldn't be being let down?
Sadly even when the DC's go to their Nan's apparently he only pops round for five minutes, sees them and clears off again. Arsehole.
How close does he live? Personally I'd stop phone contact, tell him you're not happy that he makes them empty promises so from now on he has to arrange it with you, and you don't tell them he has said that he is going to turn up. Give him a couple of opportunities to show he can be relied on, but tell him it's his last chance, you won't let him do this to them anymore, you'll wipe the slate from before now, but you're starting again, and as soon as he fucks up that's it, he'll have to go to court to get you to agree to him seeing them again.