Still only deliver 1.2v though - in a 6v setup you're only getting 4.8v. It is possible to obtain rechargeable alkalines (eg
CPC | CPC - Over 100, 000 products from one of the worlds leading distributors of electronic and related products.) that don't self-discharge very fast and do deliver the full 1.5v. They need a special charger, but the cost of charger with batteries is not dissimilar to the cost of one set of good alkalines.
I've used these a lot - the only real downsides are a) that it's better not to let them completely discharge and b) that the children seem to find them as useful as I do.