The Pegasus snap models are quite nice and rather inexpensive. Even experienced modelers have turned them into nice display models, but kids can easily build them. WWII: Spit, 'cane, 109E, 109G, FW 190A, ME 262 and P-51D, I believe.
The old Monogram 1/48 line (now Revell) would be another option. A bit more detailed and advanced, but still relatively easy. Good luck and enjoy!
Regards,
Bob