My 13y/o daughter is a type1diabetic and diagnosis w celiac disease. Is it ok to include vitamin C together with her multivitamin?

OK but why... As you are aware, "multivitamin" already includes vitamin c. It is not clear whether additional amount of vitamin c will bring any more benefit.
Perfectly OK. But you should ask her dr. About vitamin b1. Dr. Thornalley in the united kingdom has shown that diabetics excrete huge amounts of vitamin b1 in the urine. Giving vitamin b1 protects them from complications.