"Joe and Katherine Jackson want those children!," a source close to the family tells PEOPLE.
Jackson had two kids – Prince, 12 and Paris, 11 – with ex-wife Debbie Rowe. His youngest son, Prince Michael II, 7, was born to an unnamed surrogate. Although Rowe is still the legal parent of the two older children, her next step is unclear.
One Surviving Parent"Debbie sent her condolences, and said she was not going to do anything about the kids ... yet," the family source tells PEOPLE. Still, "Joe and Katherine think she's going to come after them."
Katherine Jackson's 22-page petition claims the children "have no relationship with their biological mother" and have been in the care of their grandmother, "with whom they have a long established relationship," since Michael's death. A hearing on the petition has been scheduled for August 3.
Experts in California family law say the grandmother's petition could be granted – as long as Rowe, who is the sole surviving legal parent of the two older children, has no objection. If Rowe does object, the Katherine Jackson could face an uphill battle to prove the kids would be better off in her care, no matter how close she is to them.
"[Rowe] is the biological mother and upon Michael Jackson's death, technically she has every right in the world to have custody of those kids," Los Angeles family attorney Michael Trope tells PEOPLE.
The petition for guardianship was filed in Katherine Jackson's name alone. Joe Jackson, 80, maintains a separate home in Las Vegas but also spends time at the family house in Encino.
Reporting by SARA HAMMEL
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