I've used a Grandpas treadle feeder for a couple of years and its been really easy. The chickens I had at the time took a while longer than I thought to get used to it but, having lost those to a fox, bought new chickens last weekend and they have already worked out what to do.
I have a friend with a cheaper treadle feeder and hers definitely isn't as good. Sometimes it gets stuck, which mine never has, and the magpies keep raiding it. Never seen a magpie using mine but then I think its heavier.
Maysie, I think there is something on the Grandpas feeder website about bantams. Perhaps you can set it to open at a lighter weight but then you might also get the magpie problem.