Edit: I realize I'm assuming the two nephews live together/nearby.
I'd recommend actually buying them a Core set each. That's the cheapest cost per ship, and will wind up with them each having everything they need to play - having two sets of dice is much nicer (especially for all the times you need to roll 4). If you feel the urge to add to that, pick up either a Y wing and a Tie-advanced (keeping with the cinematic experience from Episode IV), or an A-Wing and a TIE-Interceptor, which make a decent pair of agile fighters.
I wouldn't worry about trying to buy them a "tournament" style list - they'll probably be playing against each other at whatever point values they can muster, and if they stay interested, they'll surely have their own style of play they want to buy for.
Edited by -Istaril, 28 August 2013 - 04:54 AM.