This is so awesome! I just love the Haflingers, beautiful horses.
As for the stopping, I think the best way to get a good stop is to just practice a lot. Also, make it as gentle as possible. When you're schooling, go on a straight line (at the walk at first) and go through these steps:
Say "whoa," or halt command at choice, nothing else, just the vocal command.
If that doesn't work, sit back. Just sit back.
If that doesn't work, LIGHT rein contact.
If that doesn't work, progressively more rein contact until you get a halt. Then release your reins and big big pets.
Keep going through this process until all you have to do is say the word. Do it at all gaits. Make sure your horse isn't on his forehand, as that will make a good halt difficult. That is what I did with my OTTB, and he comes to a complete stop at all gaits just from a strong, enunciated "woah."
Hattie does respond well to voice commands and when out galloping all I have to do is say woah and she slows which is lovely, but in the dressage ring I mustn't speak :) She does respond well to half halts and goes with a light contact, but I mess things up when trying to get a square balanced halt.