Your kids walk through the airport with their helmets on???? Is that street cred? My kids might have done that a couple of years ago but I think now they'd feel a tad silly. They wear heelies in airports (helps with getting from A to B quicker) but not helmets!
I came back via easyjet last year and bought my ski boots in a boot bag back through with no problems back into Gatwick. To be honest I would check with the airoport.