My DS is 5.4 and 19.5kgs which I think makes him just on 3 stone as well. Is that really that light? Out of complete interest the other day I looked up his red book and for a boy he was spot on the 50th centile - so I think for a girl it would be above 50th centile. DS is on the 90+ centile height wise, so he does look very skinny, and there is a big disparity between his height and weight, but he is healthy and lively and has good skin and hair and doesn't get colds or run down that often, so we are just living with it.
Like your DD he was a very "bonny" baby - in fact as a toddler he had exactly the same dimensions as a small boulder. He also has a large bowl of porridge with maple syrup and 2 slices of toast with lemon curd for breakfast, a good packed lunch and a full, hot tea after school. I'm pretty sure with him he is in a metabolic/growth phase where the ampount he is eating is only enough to keep him going but not enough to put weight on him.
DS is allowed a reasonable amount of home made cakes (flap jack, banana bread, carrot cake, oat cookies etc). Peanut butter in things or on wholemeal toast is also another winner. Interestingly he will often say he is full and, for example, leave half of his pudding. In the long run I think that recognising and obeying your appetite is a good thing, even though he occasionally could do with the calories.
I'd keep the behaviour things completely separate. Sounds like you are doing all the right things.
One thing, what build are you and your DH? although I am relatively chunky, DH is like a whippet and DS may just have inherited this.