The Time Garden (Tales of Magic Series #4)

When wishing for magic, it's hard not to wish for too much. If Jane, Mark, Katharine, and Martha had stopped to think--oh, if they had only stopped to think!--they would have ordered magic by the pound, or by the day, or even by the halves as they had in Half Magic. But no, they asked for magic by the lake--and now they have to deal with a whole lakeful of enchantment! Soon the children are awash in magic. They find themselves cavorting with mermaids, outwitting pirates, and--with the help of a cranky old turtle--granting a little magical help to the one person who needs it most.