Of course, courts can do whatever they want, but... The children are habitually resident in the US for well over a year now; CC did not abduct them, she had a court order allowing her to move them to the US (sorry, Otto); the children have no real connection with Canada as they only lived there for a few months; all evidence of their current health, education, and welfare is in the US, not Canada; and I believe CC filed for divorce, and I would presume custody, and a restraining order in her current location in the US.
https://www.cbc.ca/amp/1.5145716x
The judge also seems to think favorably of CC:
The judge was presented with wildly different stories by Boyle and Coleman, but ultimately decided that it was in the best interests of the children to allow them to return to Pennsylvania with their mother. “Under the exceptional circumstances of this case, requiring C.C. (Caitlan Coleman) and the children to remain in Ottawa would be akin to once again holding them hostage.”
...
Justice Engelking, however, said she received no evidence to support the contention that Coleman suffers from a mental health issue that could affect her suitability as a parent.
“The court does have evidence, on the other hand, that C.C. is healthily and protectively parenting the children,” Engelking noted before concluding: “Based on all of the evidence before me, I can find little to suggest C.C. would not be a suitable temporary custodian of the children.”
Ex-hostage American Caitlan Coleman accuses Canadian husband Joshua Boyle of abuse, in court documents
My guess is that the Ontario judge will defer to the US court handling the divorce, where CC and the children have lived for the past couple of years.
Child Custody Jurisdiction in Canada | The Law Office of Jeremy D. Morley
BBM
c. Ontario
-In Ontario, the rules are contained in Sections 22 and 23 of the Children's Law Reform Act. Those provisions are as follows:
Jurisdiction
22. (1) A court shall only exercise its jurisdiction to make an order for custody of or access to a child where,
(a) the child is habitually resident in Ontario at the commencement of the application for the order;
(b) although the child is not habitually resident in Ontario, the court is satisfied,
(i) that the child is physically present in Ontario at the commencement of the application for the order,
(ii) that
substantial evidence concerning the best interests of the child is available in Ontario,
(iii) that
no application for custody of or access to the child is pending before an extra-provincial tribunal in another place where the child is habitually resident,
(iv) that no extra-provincial order in respect of custody of or access to the child has been recognized by a court in Ontario,
(v) that
the child has a real and substantial connection with Ontario, and
(vi) that,
on the balance of convenience, it is appropriate for jurisdiction to be exercised in Ontario.
R.S.O. 1990, c. C.12, s. 22 (1).m