The kids themselves don’t sin by missing Mass because no one can drive them.
They are 8 and 9, so past the age of reason. So the obligation to attend Mass is true.
I do not agree with some here who say the mother does not sin because she is doing something else that is good.
. It is her responsibility to get her kids to Mass.
The retreat is not an emergency, sickness etc. and is not obligatory.
She should find someone else to drive her kids, leave the retreat early, go to a different Mass at closer Catholic Church nearby,
or obtain a dispensation.
If I am wrong please cite authoritative Church documents.
Missing Mass is grave matter. Causing your own children to miss Mass because you voluntarily want to do something else (without dispensation) would also have to be grave matter.
I am sorry if it sounds harsh but she asked what is the legality of her situation.To just say it’s OK you are not sinning without any documentation is a bit scandalous.
CCC:
No. 2181: “The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants). Those who deliberately fail in this obligation commit a grave sin.”