As other posters have noted, it's extremely rare (here in U.S.) for picu or nicu trained docs to do some "primary care on the side"-- I think being a specialist in peds specialty that actually does clinic (cardiology, nephrology, basically anything but PICU or NICU) would lend itself better to doing a day or two a week of community peds. The issue with this is that every day you aren't working in our specialty is a day less that you aren't advancing yourself in your specialty-- if you're at an academic center, this can be very important. I think you'll also find that after doing a PICU or NICU fellowship, general peds is the last thing you're interested in doing, and it is VERY hard to balance NICU/PICU schedules/call with a clinic practice. There ARE ways of doing community work without being a general pediatrician part-time. For example, you can volunteer with community initiatives, education in schools, etc. You can also do work abroad in post-op ICU care in developing countries when surgeons and anesthesiologists go abroad to donate their services. As the picu doc on the team you serve as the cardiac ICU attending for children in a developing country who are getting no-cost vital surgery( hearts, cleft palates/lips, etc.).
It sounds like you're early in your career-- I would be wary of mentioning on interviews for residency/fellowship etc. that your main goal is to be an intensivist who does some general peds on the side. These are traditionally on opposite ends of the spectrum and for someone who doesn't know you may be a red flag for lack of focus. focus on talking about an interest in community health and service, volunteer work.
Being a general pediatrician takes a lot of practice. The best general pediatricians, as with any profession, do it day in and day out and they are the best pediatricians out there because they do it all the time. Same goes for PICU/NICU-- diluting one job with another completely different one makes it harder to become an expert in that field. Besides, you will have plenty of family members and friends who hit you up for general pediatric advice all the time-- and you'll realize if you're an intensivist a lot of the time you don't know the answer about this vaccine or that oral antibiotic because that's just not your expertise anymore.
long post, hope this helps- great intentions..just food for thought. good luck!