Let's take a little longer look at the Chapter and see if it gives any more insight into Jesus warning.
All Scripture is quoted from the King James translation unless otherwise noted.
Matthew 18:1 through 6 At the same time came the disciples unto Jesus, saying, Who is the greatest in the kingdom of heaven? And Jesus called a little child unto him, and set him in the midst of them, And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven. Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven. And whoso shall receive one such little child in my name receiveth me. But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and that he were drowned in the depth of the sea.
What exactly is Jesus telling his disciples with these statements?
Jesus only used the child as an example of how a Child of God should be.
Children are unassuming, non judgmental, trusting, and loving, and unconcerned with the things of the World.
Note:Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven.
Jesus is saying that that is the way they should be, and place their trust in him, from this point on Jesus is referring to the converted and not the child.
In the next section Jesus is warning them about not being a stumbling block to other believers.
Mat 18:7 through 9 Woe unto the world because of offences! for it must needs be that offences come; but woe to that man by whom the offence cometh! Wherefore if thy hand or thy foot offend thee, cut them off, and cast them from thee: it is better for thee to enter into life halt or maimed, rather than having two hands or two feet to be cast into everlasting fire. And if thine eye offend thee, pluck it out, and cast it from thee: it is better for thee to enter into life with one eye, rather than having two eyes to be cast into hell fire.
This warning is very clear in that he is telling them that no matter how hard it is they must overcome any tendency to cause another Christian to sin. That even includes the unintentional things.
Now to the point of your question.
Matthew 18:10 Take heed that ye despise not one of these little ones; for I say unto you, That in heaven their angels do always behold the face of my Father which is in heaven.
The warning here becomes more clear when we realize that he is referring to Christians here and not children. What he is saying is there should be no jealousy among Christians and we should never assume a holier than thou attitude toward, other Christians.
Jesus alone is qualified to judge the holiness of anyone!
As far as the guardian Angels part that means that as a Christian we are always in the presence of God.