For the love of revenge on girlfriends
This is a good place to start since childhood particularly inspires fun and innocence. Toys, cartoon characters you used to watch and other childhood fun stuff are where you can find such cute names. Then there is your memorable times category. This is particularly easy because all you have to do is to remember a wonderful time you shared together as inspiration. For example, if you went to the park and she kept looking and touching the flowers, you can call her bumble bee.