You could hand deliver them in a sealed envelope but it won’t stop the recipient scanning them then sharing them on messenger, texts etc.

You’d need to consider where and how they get shared beyond the person you send them to, to then decide which level of privacy is appropriate. Ultimately, even though others don’t recommend WhatsApp (nor would I) - it’s maybe the best option in this case. Accessibility, ease of sharing just no guarantees on the encryption because the source is behind closed doors.