Changing out background to sunset

[LindaBig]

I would like to take a picture of my two nephews and change out the background for a beautiful sunset, however, the lighting on the boys and the sunset isn't the same. How do I make it look authentic?

[kImages]

Linda - please post both images as it will help the forum members in offering suggestions. Someone may even jump in and give it a go and explain their methods/results.