The triple-boards are about 1/2 pediatrics, 1/2 psychiatric. They are about 5 years, with the first half mostly peds and the second half mostly psych.
Its been quite awhile since residency and general practice stuff, but I think general behavioral guidance is the job of a general practice pediatrician. Likewise, I think ADHD management, at least the more milder cases, are within the scope of a general pediatrician. Beyond that, I have seen pediatricians do depression, bipolar, psychosis, etc, however in my limited experience its mostly medication based without actual therapy and it didn't go well in the limited cases I've seen (super aggressive behavior, successful suicide attempts, etc.). Again, being in PICU, I only see when things go wrong, so take my experiences with a grain of salt.
As far as moonlighting, I suppose it would be possible, but it would likely be based on how desperate the need of the system is. There is a lot of child psych in the ER (from what I remember) that is triaged by social workers and nurses (eg admit to psych hospital, outpatient follow up). I suppose you could find a role in that capacity, but in general, I doubt you would be able to do much more than triage (well, you could order emergency psych drugs, eg Haldol). Usually speaking from a hospital standpoint; no board certification, no job. If it was your own practice, you could offer therapy as you see fit, but if you don't have credentials for the treatment you provide, you open yourself up to litigation if something goes awry.
If it's something that you really want to do, I would suggest getting the proper training.