Who are the "little ones" in Luke 17:2? From Luke 17:1-3, NIV:


  Jesus said to his disciples: "Things that cause people to stumble are bound to come, but woe to anyone through whom they come. It would be better for them to be thrown into the sea with a millstone tied around their neck than to cause one of these little ones to stumble. So watch yourselves."


One common interpretation of this passage is that the "little ones" refers to children. However, that is not obvious in the context, and there are many other possibilities, including sinners, doubters, everyone, and so on. Indeed, its meaning may also hinge on what "to stumble" means here: whether it is to doubt their faith, to tempt to sin, or something else.

Is there a common agreement on who the "little ones" are in Luke 17:2?
Matthew 18 also cites Jesus' words regarding little ones, and does so more frequently:


  At that time the disciples came to Jesus, saying, “Who is the greatest
  in the kingdom of heaven?” 2 And calling to him a child, he put him in
  the midst of them 3 and said, “Truly, I say to you, unless you turn
  and become like children, you will never enter the kingdom of heaven.
  4  Whoever humbles himself like this child is the greatest in the
  kingdom of heaven.
  
  5  “Whoever receives one such child in my name receives me, 6 but
  whoever causes one of these little ones who believe in me to sin,
  it would be better for him to have a great millstone fastened around
  his neck and to be drowned in the depth of the sea.  Matthew 18:1-5


In this context, Jesus specifically identifies a child and speaks of children, then refers to children as "these little ones".  So, interpreting Scripture in light of Scripture, it is pretty conclusive that "little ones" refers to children.