You need to consider your feelings towards each age group. You can become a clinical instructor in any of the units though I would say more likely so from Adult icu or picu.
Having worked in both NICU and Peds CICU I can think of a few things to consider.
How are you with families? Family members are going to be a much bigger presence in peds & nicu, there is a lot of teaching them and a lot of learning from them. They are another member of the childs care team and we must work together to have the best outcome for the child.
Are you good with kids? Silly question maybe but this is an important part of working in a PICU, you have to be able to identify with all age groups, neonates, toddlers to teenagers and if its a cardiac unit then even adults with congenital heart issues. You have to be a little silly, willing to do an assessment on the floor and sometimes chart with a toddler on your lap (yes, even in the ICU). More so you need to understand developmental levels and how to approach painful/difficult tasks with each age group.
What's your patience like? Even in a level 3c NICU you will spend days trying to encourage an old preemie to eat, it's time consuming, frustrating and actually takes quite a bit of skill.
And then there's your own personal beliefs and ethical issues to consider. Every ICU has ethical issues. In the adult world it's the 92 year old with breast cancer and mets to every organ in the body but family refuses a DNR, in peds its the child abused right to the brink of brain death by moms boyfriend, mom won't press charges or leave said boyfriend and wants everything done for said child who, if he survives, will never walk/talk/see/hear again. In the NICU its saving the smaller and smaller babies. The "23" weekers who by the time they've been resuscitated really appear more like 22 or even less and now parents want everything done to keep essentially what was trying to be a miscarriage, alive.
Now picture yourself someday doing chest compressions, because in an ICU it will happen, what is the age of the person you are working on and does it seem like something you can handle? For me I could never do compressions on that 92 year old and feel ok about it. Sometimes I still don't feel ok about it with the kiddos, but when we have a good outcome, and many of them are, I feel satisfied that I've helped that child potentially have a meaningful life.
Don't go after something because of what you think you might want to do someday because that can always change once you've spent time being a nurse. Think about the present and what appeals to you right now. Also what is available to you. It's much easier to move around in nursing after getting that initial experience and you may find what you thought you wanted to be completely different in a few years.
llg is also right in saying that there are few ICU focused clinicals to teach. I do have a friend of mine though who works in the peds cardiac icu and she teaches a clinical elsewhere in our children's hospital, but she's in school to be a peds NP therefore has done her own work and additional clinicals outside of the ICU setting