Even though raisins wouldn't be my choice of hypo treatment..
They can be used, as they are very high in glucose content after all they are just dried grapes.. Raisins are actually one of the recommended hypo treatments, alonside fruit juice and other similar dried fruits..
There would be little to no difference in the time they would take to adsorb, I would say it would be unwise to give to very young children as they could be a choking hazzard..
You can buy small cartons of raisins at most supermarkets, sold for kiddies lunch box's, advantage of this, is that they will have carb count on the nutritional info, and nice small to carry and easier to keep fresh until needed. You just need to ensure that they are enough to treat an hypo and not too many to over treat..