What if one of the children had decided to take off from the mobile screening unit - what should the other radiographer have done in those circumstances? Run after the child - and the units are usually in car parks - and risked being accused of 'grabbing' him or her and then be suspended from work with an enquiry, or call for the grandmother who may have been in a state of undress?
It was kind of you to offer to look after them but she shouldn't have taken them with her. They should, however, make it clear on the appointment letters that children should not be brought to appointments.