2 strokes for life said:
I would go for eaither bike. The DRZ400 is a little harder to ride in tight trails due to the weight but still a good bike.
Actually, the DRZ-400 is only a few pounds more than a CRF-450x or WR-450.
The DRZ-400 got a reputation for being heavy when every one else was riding 215lb 2 strokes. Suzuki does list its weight (the 400e model) at 265lb. Which is only 5 more than a XR-400. But the DR has liquid cooling and e starting, plus it's more powerful than a XR-400, so there's no comparison.
But beyond all that, it's a great trail bike. Smooth power and very reliable.
Now, I cannot even begin to count the number of times when I caught up to a guy on the trail riding a 450 something following Jr. who's actually riding a pw or crf 50. Honestly, it had to suck for both. Because pop's is stuck in first and slowly burning up the clutch while Jr. is getting the brunt of the 450's exhaust note.
So in your case (and a couple others), do yourself and Jr. a favor and stay away from any moto bike. If he's going to be on a Honda 50, then get a CRF-230 or 250x. Or if you get him a PW-50, then a TTR-225/250 or maybe even a WR-250 would be a good choice. And if he's big enough for a DRZ-70 (now with electric starting!) then consider a DRZ-250.
That way you'll start bonding with Jr. right away, because "he rides the same bike as dad!" And at that point your really starting to build a father son relationship that many of us only dream about having.