An employee complains about being called “darling” I told her male Manager that she should grow up and not worry about it as it was a compliment!!!!!
You are probably facing a claim for sex and age discrimination including harassment on both counts. Unless you are able to argue convincingly that you call everyone darling including the men you are on weak ground and the law says anyway that harassment is defined by the ‘victim’ not the perpetrator. The basic premise of harassment is that it is unwanted conduct that is offensive to the recipient. The claim for age discrimination/harassment is on the assumption that this person is young. It should much easier to claim for multiple harassment due to the Equality Act 2010.
The best thing to do would be to apologise immediately and promise not to do it again to them or around them as it is possible for someone to claim for harassment they have observed.