I have to say I played Mozzie but then decided I wanted to play Wonderful Tonight. Yes Wonderful tonight was in my opinion harder and it took me a lot longer to get that to the standard of Mozzie, but now learning grade 3 pieces is a lot easier in my opinion. Also wonderful tonight is a lovely tune. Although if your nephew is struggling then maybe Mozzie would be a good idea as its really catchy and fun to play!
Never played snowdrop but Mazurka was great fun and not too hard to play
I don't know though i'm 16 so maybe my thoughts are different
What does your nephew want to play?
If he really hates the pieces then it is likely he will find them harder. Although if he wants to play Wonderful Tonight and Snowdrop then mabe if he works hard he could do it.
I found learning wonderful tonight in stages was easiest. Mostly spending hours playing the left hand over and over on its own
Good luck