What you are looking for is essentially the holy grail of medicine: great lifestyle, good pay, flexible schedule, interesting and rewarding work, etc. Accept now that anything meeting these criteria will be very competitive and require exceptionally dedicated work on your part to achieve. Yes, this will mean a strong performance in medical school followed by strong work in residency.
All of the residencies and fellowships for any of these things will require long hours. 80 is a bare minimum, especially when you're a more junior resident and depending on the program and the service, could easily apply to when you're more senior. As mentioned above, all the research and outside projects must be done on your own time beyond the 80-100 hours you're working.
I'll put in a plug for ENT as a great way to work with kids. Most non-fellowship trained community ENTs devote a large portion of their practice to treating children -- ear tubes and tonsils are two of the most frequently performed surgical procedures in the US. Residency is usually 5 years and all programs have you rotate on pediatric services, with some having more than others.
With only a few exceptions, the full surgical breadth of adult ENT procedures are also performed on children, though straying too far beyond the bread and butter means you would have to do them at a children's hospital and would therefore probably need additional fellowship training to be credentialed. With 2 exceptions, Peds ENT fellowships are all 1 year in length and most not nearly as competitive as their Peds General Surgery counterparts.
Opportunity for international mission work is virtually unlimited. There is a huge need in the developing world limited only perhaps by the infrastructure necessary to do the cases.
From med school to completion of training, budget about 10 years start to finish for any surgical field. ENT fellowships overall are not that competitive compared to other fields, but ENT itself definitely is and is currently one of the more difficult specialties to match. Successful candidates tend to be near the top of their class with nearly flawless grades and board scores along with significant research and meaningful EC work during medical school. Hours wise, this typically means studying longer and harder than most people. For some, they end up electing to take a year off between 3rd and 4th year to do research and this seems to be getting more and more common.
Residency is 5 years like most surgical fields. A handful of programs have 7 year research tracks that include 2 NIH-funded years of research, but the overwhelming majority of them are 5 years. Hours will vary depending on the program and the service. In general, count on them being pretty brutal though not nearly as soul-sucking as some other surgical fields. 60 hour weeks would be a really light week with no call (6am-6pm M-F); add in one call night, a weekend, and some longer days and it is VERY easy to hit or exceed 80 hours. In reality, the number of hours isn't as big an issue as the unpredictability of them and this is something that sets the surgical fields apart. If a surgical emergency rolls in 15 minutes before you're "off," you may not be going anywhere for awhile.
Hours in practice can vary widely depending on how you set up your practice. If you're doing bread and butter peds cases for affluent healthy children in an outpatient center, and you have good midlevel support and a large group practice in the call pool, you may have a pretty sweet lifestyle. The sicker and more complicated your patients get, the more hours you work and you're more likely to get called in for emergencies. Judging by the number of transfers we get because the hospital didn't have an ENT on call (in communities where I can name 2-3 local ENTs), some people must be living the 'Early Nights and Tennis' motto. But that definitely won't be you as a resident!
So in summary:
~10 years school and training
Dedicated hard work start to finish
Long hours in training
Possible option for more friendly lifestyle in practice if you're flexible on money, location, scope of practice, etc.
