OPERATION Santa Claus, the RTHK Radio Three/South China Morning Post annual charity drive, paid dividends yesterday when 34 children from the Braemar Hill Nursery School travelled to Tai Po to meet 23 mentally handicapped children at the Pinehill Village Pre-school Centre. The Pinehill pre-school is run by the Hongkong Association for the Mentally Handicapped, beneficiary of Operation Santa 1992, which collected $4.25 million. When the Braemar Hill children arrived at Pinehill yesterday, they decided there was no difference between them and the pre-school children. ''Both lots of children had a very good time, because they mixed very well. Some of the Braemar Hill children can speak Cantonese and they got on very well,'' said Miss Nora Wong, the association's general secretary. ''This is the best part of Operation Santa. I hope it is just the beginning. The interaction between the children means they won't be afraid of our children when they grow up.''