Mary Pope Osborne 
Magic Tree House 11: Lions on the Loose 

Whisked away to Africa on this adventure in the tree house, Jack and Annie cannot believe all the wild animals they see. They end up showering with elephants, helping wildebeests cross the river and they have a picnic with a maasi warrior! But when their route back to the tree house is blocked by some scary lions they need to come up with a clever plan of escape – and fast!