I'm sure Alpha are different from JWs but they still had the same feel to me. Initial visits to a KH are often warm and friendly etc etc...
With regard to DAing, I'm not sure how you see yourself currently being connected to the name Jehovah if you've already left. You know that your hardcore family will read your DA in whatever way they wish - evidence of pride/arrogance or whatever that they've always seen in you and I never heard of any witness that DAed or was DFed that made me "think" while I was a JW. It was my old pals that had left, avoided being cut off and got back in touch with me after some time that aided my departure. Not through de-culting me, but through being normal, having moved on and being supportive and non-judgemental. If they had been DAed I would not have let them back in to my life when I did
It's early days for you and I'm hoping that I don't sound patronising but I'm suggesting that you think ahead. It's nigh on impossible to send a clear message by DAing, they see what they're told to see.
By letting your kids see that you can change your mind about something and still be a lving dad is more important than letting them know where they stand. Life, the world, is wonderfully messy and complicated and making a stand is sometimes not the best move.
"Shut up and eff off Euan" yeah, i hear you!