The kids are cute and the pose is nice. I like the tousled hair and apron - they are kids, not models and it makes it more natural.
From a critique side of things ( and I have to add the disclaimer that I haven't tried portrait photography yet so I have limited insight and less right to say much), it would be nice to have a bit more oblique lighting, rather than straight on flash - it would add depth to their faces. It seems that your granddaughter is a bit further back than your grandson, so given a shallow DOF, her focus is a bit soft. Moving her face into the same plane as his would sharpen her image.
(Being nit picky, I also wonder if you took several shots and they were posing for a while - maybe it's his normal smile, but it seems that his upper lip was getting tired and he is starting to curl it in.)
Still one that would look nice on wall.