I wouldn't crop it - it's just about perfect. You might have been able to lose that blank sky by moving slightly to the right, but you're stuck with it now. Draw a freehand mask for the whole of the background above the bench, keeping the mask tight around the kids heads. Then feather the mask by about 14 pixels and adjust the histogram just for the background to reduce some of the brightness and glare. While you've got the mask still drawn, experiment with the gamma and colour saturation of the background too.
It's a lovely shot and one you can be proud of, but I don't think that frame is doing anything for it. All the above work is easier without the frame anyway, so I'd crop the frame out before you start working on the pic and add something more suitable afterwards.
I'm sure the parents will be well chuffed with it anyway.
