There aren't any obvious simple ones - if you Google - "advent prayers" children - there are a number of links to American (usually Catholic) sites which have some interesting prayers, but they're not in simple language necessarily. There are some good craft and activity sites as well.
Are you wanting them for school (as a teacher) or as a parent? For lighting advent candles on a wreath - so one for each Sunday? If you look up the themes for each of the Sundays, it's not impossible to make up a little prayer around that theme: for John the Baptist, for Mary saying "yes" to God, etc. It might depend on what bible knowledge and religious language your children are familiar with. We have a set of prayers we use for the lighting of Advent candles on a wreath each Sunday, but the book is not to hand at the moment. I'll look it out.