Since I've recently completed all of this board stuff, I will chime in. I did the integrated child fellowship (3+2). F1 year counted as PGY4, so F2 year I was eligible to take the adult boards. I took it, passed, then went on to complete fellowship. My fellowship program really bugged the hell out of me to register to take the child boards the September after graduating, but I was honestly sick of all that stuff for a bit. I knew I was leaving fellowship to go into a full-time child psych position, so I wasn't really worried about waiting. Plus I was graduating the program, so I didn't really care if they were upset about it.
I took the child boards this past September, just under 2.5 years after completing the fellowship. Being board certified in general psych is a prerequisite to taking the child boards. One is considered, "board eligible", for a certain number of years after completing all of the requirements to be such, and you can technically take the exam at any point in that timeframe. I don't remember exactly how long it is, but I think it's 7 years? 4 years? I don't remember.
I didn't find either the adult or the child boards particularly difficult, and I was able to pass without really studying for them. However, I have known I can do this type of thing since high school and that's been my MO my entire life. I thought both exams weren't very well written and the content did not seem to be on par with our level of training and education. Large portions of the child exam seemed to be undergraduate-level learning theory and research design type questions. There were so many questions involving rats.