Three cheers for you, Elizabeth, for taking the lead on getting the kids out backpacking! Too often, scouting omits the outdoor stuff. My wife was a girl scout leader for many years, and it was shocking how little exposure some of the girls had had to any outdoor activity. E.g., some of them only wanted to drink store-bought bottled water, and refused to drink tap water. Parents also tend to generate a lot of anxiety, which can get passed on to the kids.
In the Los Padres area, you may want to check whether you're going to be in an area that was affected by the 2008 fire (map: http://www.kusp.org/fire/images/map_basin_7_14_large.jpg ). I did a springtime trip in the Ventana Wilderness this year (possibly somewhat north and west of your area?). After the fire, the thick chaparral vegetation regrew and obliterated a lot of the trail system, which will only be restored over a period of many years. I don't know about the area you'll be in, but in the area I was in, there was so much brush growing over the trails that wearing shorts would have been a real invitation to suffering. I'd at least make sure that the kids bring long pants that they can change into if necessary. They'll probably resist this, so educate them a little in advance about what chaparral is, and why it's called chaparral. I'd suggest not telling them that pants with zip-off legs are required, because that's just another expensive item for the parents to buy, which the kids will rapidly outgrow. Just have them bring one pair of long pants and one pair of shorts.
Isn't rain a pretty remote possibility in that area at this time of year? On the off chance of rain, garbage bags would be a more compact and lightweight alternative to the 99-cent ponchos. Hiking in wet cotton socks is miserable, but good wool or synthetic hiking socks are expensive, so I'd suggest not presenting them to parents as a required item. Or as a compromise, possibly tell them to bring two pairs of socks, at least one of which should be non-cotton.
Wide-mouth nalgebe bottles are lame, IMO. They're heavy, and they tend to leak. Instead of a 1-quart bottle, I'd suggest having each kid bring two half-liter water bottles, the type you buy bottled water in. Most families will have these around the house, so it will also save them money. You can alternate bottles, i.e., when you come to water, you have the option of refilling one and putting in the tablets while still being able to drink from the other. There are a lot of urban folktales about dehydration, which may cause people to carry way more weight than they need ( http://ajpregu.physiology.org/cgi/content/full/283/5/R993 ,http://jasn.asnjournals.org/cgi/content/full/19/6/1041).