Why did so many people take part in the crusades not only knight and solders but also ordianly people and children?

Deus Vult! (God wills it!)

Some: for land.

Others: for past sins to be absolved.

Others: pure religious motives.

Others: no future at home (second sons, etc).

Children: Naively thought that their innocence would convince the Muslims to give the Holy Land to the people of Christ (during the Childrens' Crusade). Instead, most starved to death walking to the Holy Land or were sold into slavery by Christian Italian merchants.