Magic Tree House

Based on The Magic Tree House series, award winning series of children books written by American author Mary Pope Osborne.
Jack and Annie are young siblings who find a magic tree house on their way home after chasing a weasel. By using the books available on the tree house, they are able to travel through time and location. The two end up with the difficult task of collecting four medallions, each one hidden in a different place and epoch, in order to help a magician named Morgan.
(Source: Anime News Network)