Have you compared with App Ski Mountain? People rave about it for beginners and kids, and that's where I plan to start my 3-year-old sometime in the next month. Buddy from Raleigh just took his kids who are similarly-aged to yours for the first time and said the rates for lessons were awesome.
I've only been to Massanutten in the summer, but I was surprised it was a ski resort. Felt a lot more like a hill than a mountain.
That said, I do know people who go with their kids and enjoy it, so I'm sure it would be sufficient for your beginners. I would just compare it to pricing at Wintergreen and App Ski Mountain before driving farther to get to Massanutten.
Also, I would strongly suggest you steer your son towards skiing. It's much easier for beginners, and if he wants to learn to snowboard later he can, but skiing will be valuable for him his entire life. You never see anyone over 50 snowboarding, and that's not just because it's a newer snow sport. It's a bigger pain to get on and off the lift, to navigate flat areas, and everyone else on the mountain hates you.