I left my ds (20mths) in a creche at a sports centre recently. When we arrived one of the staff just sort of took him off and he seemed happy and engaged.
I thought that by saying good bye I might just be drawing attention to the fact I was going and disturbing him. So I slipped away. Though when I got back just over an hour later he was upset and apparently had been asking for mummy.
So maybe I should have explained to him that I was going -obviously with an older child I would have done - what do others do??