Any advice on being a neonatal or pediatric nurse?

I'm not sure if you're looking for advice on how to become a neonatal/pediatric nurse, or if you're looking for a description of what it is like. I'll give you a little bit of both.
If you're looking to become a neonatal/pediatric nurse, my advice is to get your feet wet as best you can prior to going into it. If you're still in school, see if you can get a neonatal ICU rotation or a practicum in pediatrics. If you're already working as a nurse, see if you can volunteer for a while or shadow for a bit.
As far as what to expect in pediatrics/neonatal nursing. I am a NICU nurse and I have worked in a level 3 NICU (highest acuity) for 2 1/2 years. In that time, I've had 2 patients of mine die, so relatively low in comparison to the number of children I have taken care of. It is still devastating when a child dies. I'd say that is the hardest aspect of the job. You have to go into it knowing that you're helping a lot of children and there are some you won't be able to help. You'll see things you wish you never saw. You will be faced with ethical dilemas, over which you will have little control. You'll have good days and bad days, but I guess that goes along with any nursing area. If you're interested give it a shot. You'll either love it or hate it. Good luck!

be prepared to be absolutely devastated when you see a pediatric patient unable to live...nothing prepares you for it, but you just gotta be able to know you have done your best. It's never a happy day, but at the least you can be prepared.

Get used to the depressing sight of teenaged mothers. Not being sarcastic, just realistic. If you work in a hospital, you'll see more teen mothers than you thought possible.

It's not for everybody. I love kids, but pediatrics wasn't for me. I can deal with bad things happening to adults, but had a hard time with kids having bad health problems. But one good thing about nursing, it one specialty doesn't work out for you, it isn't that hard to move into a different area.