LOS OLIVOS, Calif., Dec. 17 — Christmas came early for some 200 children who got to play at Michael Jackson's amusement park and meet Santa Claus. ''I hope you have a wonderful day. Merry Christmas. I love you,'' the pop singer called out to the children Friday from the driveway of his estate. Jackson, who faces trial next year on child molestation charges, appeared happy during the few minutes he spoke to the arriving throng.