This time around, I didn't bother to grill the vegetable scraps first. It just go straight into a pot of boiling water and simmered for 45 minutes or so.
The result is rather thin, the stock doesn't have as much depth as the first round. The kids still like it though so it's not a total failure.
What we can learn from here is that grilling the scraps first will give a more complex flavor. Straight up boiling is just too plain.
Maybe it's something similar to coffee beans - you need to roast it first to bring out the full flavor profile. Anyway, will test again next time.