We were surprised to find out that during the mid-week hike they got "lost" and had to use their day-packs to spend the night under the stars. [This was completely planned and was part of the scouts earning their Wilderness Survival Merit Badge.] My son had a well-packed day-pack, including long pants and a jacket.
Temperatures plummeted as it got dark (summer lows in the area average around 50 degrees). The boys put on their jackets and hats and then pulled out their emergency blankets. My son described the blanket as being way too little (he is 4' 10"). The blanket was only big enough to wrap completely around his upper body. He indicated that complete coverage was important as any gaps in the blanket allowed the cold air in and negated any warmth added from the blanket. Their scout leader also brought trash bags for each boy to use. My son felt that he really needed two mylar blankets and two trash bags to stay "barely warm." By 3:00 a.m. it was clear that the boys (especially those in shorts) weren't going to stay warm with just the supplies on hand. So they packed up and hiked back to camp where they all promptly snuggled into their warm sleeping bags.
In contrast, it was interesting to hear a cousin relate the story of his scout camp just weeks earlier. A group of 19 scouts and several leaders got lost in the Washington mountains while hiking. In that large group of scouts, only one scout had matches and food (a bag of goldfish). Temperatures dipped below freezing and it snowed that night. Thankfully, they had those matches and were able to build a single fire (too little dry wood for more). They were found the next morning. No one was permanently harmed. Unfortunately we stories like this all too often.
Always, always, always be prepared for getting lost - even on day hikes!
(and) Rethink your use of emergency blankets. If my 12 year-old son needed more than one, I'm certain that the adults in our family would need even more. I've seen the "sleeping bag" style and might give those a try. However, I honestly don't think they would work for the larger adults in our home. I think I will add several mylar blankets to each 72-hour kit and to my car kits. I'm also going to add some garbage bags to those kits.