Today is Michael Jackson's birthday ... he would have been 54 years old ... and his kids are honoring his memory with a family trip to MJ's boyhood home in Indiana.

Prince, Paris and Blanket were spotted in Gary, IN yesterday ... along with their co-guardian Katherine Jackson and their aunt La Toya. TJ Jackson -- the OTHER co-guardian -- also made the trip.

Michael will be honored at the annual "Goin Back to Indiana" celebration in Gary ... which kicks off tonight with a candlelight vigil in front of MJ's childhood home.

The kids are also expected to throw out the first pitch at a Gary SouthShore RailCats baseball game this week.

Michael died on June 25, 2009. He was 50.