FCE is neither easy nor difficult. FCE is simply a level. When you are ready for it... it is easy! If you are not ready for it... it is very hard. PET is not designed specifically for children. Many young people take it, but so do adults who are want to take their first real test. Cambridge has the Young Learner Exams for children. The style is very different from PET.

You need to get your level assessed properly. Do not waste money on exams unless you are ready. Too many students FAIL exams because they are not of the correct level. So get assessed... then sit the correct exam!

It depends on your level of English. In my opinion, KET and PET are for children. You have to have Upper intermediate level to pass FCE. If you pass this test http://www.cambridgeenglish.org/test-your-english, it will help you to decide which test suits you.