Hydration Packs made popular by CamelBak are really good for kids on day hikes.  They are small, hold enough gear for the day and are a lot of fun to drink from so the scouts stay hydrated throughout the hike.

CamelBak has lot's of different styles.  One of the smaller ones like the Camelbak Rogue

are great for small scouts as it is small and lightweight.

A hydration pack is a regular backpack that is tailored to carry a bladder full of water (see picture to the right).  The water bladders have tubes with mouth pieces attached to them so the wearer can easily drink from them without having to stop and use your hands.  The mouthpiece can be clipped to the shoulder straps so all the wearer has to do is lean over and drink.

CamelBak's are very durable and will be useful for many years.  I have had one for at least 10 years of casual use and the only thing that I had to replace was the bladder, which are fairly inexpensive.

In general I recommend this type of pack for day hikes as they are lightweight (which means less complaining) and fun to drink from so the kids (and adults) will keep hydrated.