Very durable machines, if you ride slower and put I would get the 500 because it is liquid cooled. 500 is also not as cold blooded. the 90s are pretty good machines and can be sped up substantially by removing the carb stop and clutch stop when the kids get better at riding.
2004 DVX 400
-FST 500 big bore/stroker, cams, valvesprings, revbox, ported head, Yoshimura full exhaust, Edlebrock carb, custom airbox/batterybox and headlight eliminator, elka shocks, janssen swing arm, gibson a-arms and steering stem, LSR gusset kit, AC nerfbars and bumper.
2008 KTM 525
-Rath nerfs and bumper, KTM a-arms, Twin-Air filters, Hiper wheels with razrs, precision dampener, Flexx bars, bored carb, 13:1 CP piston, 806 cam, Weapon exhaust, Renthal chain and sprockets, quad-tech hood, FourWerx parts
-2009 sponsors-661, Oneal, Works Connection, CP Pistons, SMX Optics, Amsoil, Weapon MX, Twin Air, Renthal, ASV Inventions, Steve's Sports Center, Chuck's Welding and Repair