You'll acquire skills in PICU that you'll probably never need in an adult ICU... like placing scalp IVs (

)for one. And if you can calm down a terrified three year old, there aren't too many adults you won't be able to do the same with. Of course there will be things that you own't learn in PICU: we don't see any IABPs, and our unit has only had one PA catheter in the 5 1/2 years I've worked here. But so much of what we do has a similar adult component, and if you have a good foundation of critical care education, you'll be just fine in an adult unit with an appropriate orientation.
I have no liking for hospitals that mix their peds patients in with their adults. The kids lose out on so much that way. And hospitals who have access to a tertiary care PICU albeit perhaps at some distance who choose instead to keep a small child in their adult ICU for several days, until the child is so messed up they can't be saved before transferring them really should lose their accreditation. In the last six months or so we've seen three such cases, babies who might have lived and been just fine had they been referred to a PICU sooner. They all came from the same hospital, they all had the same admitting physician and they all died. It's awful.