So, the SCOTUS sided with Hobby Lobby

[Ravenshire]

I know this is a little out of context, but can a religious guardian refuse his/her son/daughter a blood transfusion for religious reasons it it's a matter of life or death (in the USA)? Considering a child cannot consent.

I'm not sure but if they are the legal guardian (say, the parent of an adopted child, or the grandparent granted legal custody) then my guess would be yes.

That answer is (for a legal guardian) an unequivocal yes. In fact, for an adult who is unable to answer for themselves, for whatever reason, the next of kin gets to decide, even if they are estranged and people close to the patient and who actually know what that person wants are ignored, unless there's a living will.