I've had that happen. Like you, my son was Dx at 18 months, and while we haven't had many of those sorts of extreme lows happen recently, when he was little (2, 2 1/2) we had a few. He would wake up crying in the night and I'd find him low. Sometimes in the 40s. There've been only two occasions he's gone below 40; once when he threw up just after eating and the insulin he'd had for the food made him plummet (that was my first time using glucagon) and once when I had not yet figured out how to do dual wave boluses and didn't realize that fatty foods are a dip-and-climb proposition if you give a standard bolus (that was my second time using glucagon).
I guess the only lessons I learned from it were these:
1) don't be afraid of that oh-so-scary needle! I've spoken with parents who've tried to get juice in their toddler/preschooler when the kid is at 32 in the middle of the night, and when I asked them why they didn't just use the glucagon, they look embarrassed and uncomfortable and say, "We were hoping to avoid it." It looks like something from a CIA interrogation kit, but it's there to be used.
2) keep records. If you know what the child ate, how much insulin he got, and how much he exercised within 6 hours of a bad low, sometimes you can figure out what combination of foods + insulins can result in blood sugar swings. In the case of Eric's second shot, we had been doing an awful lot of walking at a fair on the day he got the high-fat meal (fried dough) and it showed me that I had to really take into consideration how fast certain foods would (or wouldn't) absorb and break down.