hmm...mind if I ask why you shot at f11? that seems a bit extreme, especially since the background in this could blur out nicely.
Seems like you missed the focus a good bit in these, it looks to be a good bit behind them.
is there any way you could have shot at another time of day? this is pretty much the worst natural light for portraits unless you're in the shade. that's my opinion, but it's shared by quite a few. it might just be the contrasty conversion you did (which is too contrasty IMO, there's blown out blacks and whites), but it looks to be around 11a-1pm, somewhere in there. try shooting late in the afternoon, you'll be surprised at how much nicer the light is. i like that you put the light source (the sun) behind them though, that is certainly better than the other way around.
i'll agree with mannella, they look like some real nice kids. i like your pose in the second one, but i think the top one, while 'cute', is a little off. especially since her clothes are extremely distracting. the boy's outfit is much simpler, which is less distracting from his face. there are some spots and stains in his clothes you probably should clone out...these pics could use some general post work, but again that's just what i think.