Excellent, and very short! article on why baptized children should partake of the Lord's Supper.
The Martin Luther quote in the comments is priceless:
"When in I Corinthians [11:28] Paul said that a man should examine himself [and so eat of the bread and drink of the cup], he spoke only of adults because he was speaking about those who were quarreling among themselves. However, he doesn’t here forbid that the sacrament of the altar be given even to children." (Martin Luther, Table Talk #365; Luther’s Works, Vol. 54 [Philadelphia: Fortress Press, 1967], p. 58)