Here are some useful links:
Kiddle
Google Junior
Encyclopedia Britannica
Indus Valley
Indus Valley - Ancient Asia
Ancient Egypt
Ancient Egypt - British Museum
Ancient Egypt - History for kids
Ancient Greece for Kids
Ancient Greece
BBC Ancient Greece
Aztecs, Mayas and Incas for Kids
Mayas
Mayan Kids
Incas
Aztecs
Aztec Pages
BBC Vikings
Ancient China - History for Kids
Ancient China
Ancient Rome
Ancient Rome for Kids
Ancient Rome History for Kids
Mesopotamia for Kids
Ancient Mesopotamia
Six Nations
Six Nations - Haudenosaunee
The Middle Ages for Kids
Famous People In Medieval Times
Kidipede - Medieval England