If someone asked me that I would panic and want to know why on earth my son WOULDN'T be intact. What has happened to him that he might not be intact? A car accident? A bear mauling? That phrasing brought to mind some sort of violence so severe it might be expected you might not make it out in one piece.
Informal, but not necessarily wrong. It probably shouldn't be used in writing - but Duo is a hardly a purveyor of formal writing style.
As I have already written in response to another comment in this formum, цел" does not mean safe. It literally means either "in one piece" or "intact" - choose the one that fits the context.
"Цел" is a short form of "целый" - an adjective meaning "whole" or "intact".
"Safe" (adjective) in Russian is either "безопасный" (if you describe someone or something that does not pose danger to you: this car is safe) or "в безопасности" (if you describe the state of not being in danger: I am safe here).