I recently managed to make this happen for a child that didn't even belong to me so you should be fine. There is an unwritten rule that you ask the CC rather than the pilots directly although even that rule seems to disappear post flight from experience.

I was on CE flight the other month and took a walk to the rear galley to see the CSM whom I'd met before and wanted to say hello to. On my way I heard a little girl asking her parents about wanting to fly the plane and her mom told her that people cant do that anymore. After my little catch up with the CSM and the econ cabin cabin crew I pointed her out to the CSM and said I think she may be a young AVGeek in the making. Left it with him. When we prepared for descent the CSM asked if we were in a rush on arrival and if not maybe hang around which we did. Once most the pax had deplaned the girl & her parents came along the cabin and as they went to get off the 1st officer knelt down and told the little girl he heard she liked airplanes and asked if she could help him on the flight deck - que yelping little girl and gushing parents.

I've only recently moved to BA & had only met that CSM once before. If I can manage it for a child I dont even know then i'd hope that unless the flight has been heavily delayed arriving or is racing to meet it's push back for some reason that the crew will make it happen.