The TA is being illogical. What exactly does she think the safeguarding risks are? I'd genuinely like to know.
If it's child abduction, apart from the fact they are so terribly rare, you'd have to be pretty dim to attempt to abduct a child from a locked car in broad daylight in a school car park at pick-up time. And even if you did, only Houdini could manage that in 30 secs.
If it's danger from other vehicles, there's more likelihood of something happening to your child whilst navigating people, other vehicles, etc. outside of the car than there is strapped inside. I was told by a PO when my children were small that it was safer to leave them strapped in the car when paying for petrol than take them with me across the station forecourt. Makes sense.
Yes okay another car could hit your vehicle in the 30 secs you were away from it but if you were standing right next to it with your child in sight it could also happen, or it could happen while you were in the car with your child next to you. There's a far greater risk of something happening outside of the car while navigating the car park although still pretty unlikely.
Our kids are all adults now but when they were little everyone I knew would leave their children in the car if they were asleep after a trip out - they have all survived without being abducted or the car bursting into flames. Every single one of them and their siblings too.
Far more danger crossing roads, negotiating other moving vehicles, walking across busy car parks.