If you allow the children to leave for a visit to their birth mother, they run the risk of not being allowed back into Canada. Actually, my biggest concern would be that she would refuse to return them to Canada...
I believe that if they were refused entry back into Canada it could invalidate the original application for PR (inland processing) but could possibly be changed to the children being processed overseas.
Please see: http://www.cic.gc.ca/english/resources/manuals/ip/ip08-eng.pdf
5.28. Applicants who leave Canada before a final decision is taken on their application for permanent residence
An applicant's departure from Canada after the application is stamped as received or after assessment of eligibility for membership in the spouse or common-law partner in Canada class may affect their ability to become a permanent resident.
A foreign national becomes a permanent resident, if following an examination, it is established that they meet the selection criteria and other requirements applicable to that class as per R72(1)(d).
Foreign nationals are not provided with any guarantees that they will be allowed to return to or reenter Canada. If they are unable to do so, their application for permanent residence may be refused because they are not cohabiting with their spouse or common-law partner at the time the case is finalized [R72(1)(d) and R124(a)].
It may therefore be appropriate to counsel applicants who are outside Canada to withdraw their spouse or common-law partner in Canada class application and submit a new application for a permanent resident visa to the CPC-Mississauga (CPC-M).
There is a lot of information on Dependant Children in the manual which you could read.
5.10. Accompanying dependent children
For purposes of in Canada processing, accompanying dependent children are children listed on the application who are applying for permanent residence and:
• reside in Canada; or
• reside outside Canada, but will join the applicant if permanent resident status is received.
5.11. Dependent children living outside Canada
For dependent children living outside Canada, the CPC-V will forward a copy of the permanent residence application listing the dependent children and their relevant contact information to the responsible visa office for verification of the relationship, once the sponsor and applicant have been assessed and approved against eligibility requirements.
The visa office will determine admissibility of dependent children outside Canada, conduct interviews if necessary and advise the CPC-V of the outcome by updating the electronic record. The CPC then informs the CIC responsible for the applicant's place of residence that permanent residence may be granted. Once the CIC has granted permanent residence to the principal applicant, it will notify the visa office, which will issue permanent resident visas to the overseas dependent children who are seeking permanent residence.
If their mother refused to return them and then refused to take them for medicals etc., they would not be granted PR visa's and in fact it could make them inadmissible.
5.12. Dependent children in the sole custody of a former or separated spouse/common-law partner
The Regulations create an exception regarding the admissibility requirements for principal applicants when their children are in the sole custody of a separated or former spouse or common-law partner. Applicants must however provide documentary proof of the custody arrangements.
The CPC or the CIC may insist on a child being examined if it believes that custody arrangements are not genuine, but rather, that they were entered into in order to facilitate the applicant's permanent residence in Canada by hiding the child's inadmissibility. If an applicant lists a child in the sole custody of a separated or former spouse or common-law partner, it is appropriate to inform applicants that:
• children who are not examined cannot later be sponsored as members of the family class, despite any future changes in custody arrangements (see Section 5.26 on excluded relationships);
I think under the circumstances, it would be far better to wait until the children have their Permanant Residence before allowing them to return to the US for a couple of months.
I am sure other's will be able to advise you.
