hmm, good question ... I've been having similar thoughts about when to start petering off bottles with DS, as he *hates* sippy cups, but is too little to understand yet. We've had some success with a straw, but it comes and goes!

I think the Thomas cup only for milk is a great idea.

Does gentle parenting exclude other types of bargaining? Eg: we're going to read a story/play a game/have snuggly cuddles on the couch together ... after you've had your cup of milk ... ?

And can you have a cup of milk at the same time? Even better - daddy doing it, showing how big boys have their milk?

Those are things which I'd be thinking about, but I'm not sure whether they're outside of the scope of your parenting preferences

Good luck!