'Sana all' is Filipino youth slang, short for 'sana all maka-experience' — roughly 'I hope all of us get to experience that'. It is used playfully to comment on someone else's good fortune, often in the comments of a happy couple's photo or a friend's holiday post. Tone is gently envious and humorous rather than bitter, similar to the English 'must be nice!' or 'lucky you'. If your child comments 'sana all' on a friend's post, they are joking about wishing they had the same luck. No concern — it is friendly self-deprecation.