Attending SSPX school is the same as attending any other non Catholic school.
Do your best to not have them go to Mass at all.
Failing that, they must under no circuмstances receive the sacraments from SSPX priests. This would be a grave sacrilege.
Treat their Masses if you have to attend, like you would going to a funeral or wedding in the SSPX: be present but not participating.