I haven't done peds hospice specifically, but I have done a lot of volunteering with terminal kids in other outlets.
Every kid is different (and you will likely run into kids with developmental delays, which means age won't tell you everything). What's nice, especially about the younger ones, is they're so in the moment. They can be hooked up to every machine you've ever thought of and still want to play hot wheels or beat your butt in Uno. Depending on how long they've been sick/how medicalized they've become and their overall personality, they may be very comfortable with a large variety of new adults or very frightened. I've found that with withdrawn kiddos (as opposed to frightened), playing alongside them often draws them into play after a few minutes. They realize that you're there for fun and not there for pokes. I'm not sure what your role will be exactly, but let the kid be your guide in how to approach them.
As mentioned above, be extremely cognizant of how the family and child talk (or don't) about health situations. I've honestly found kids don't talk as much about medical situations as adults do - again with the being extremely in the moment - but it may come up. You may get more chatter about it from siblings, but again, you want to be sure you're using terms they've heard and not introducing new information that's not coming from the family.
Teenagers can run the whole gamut. I've known kids who were extremely angry and others who were more at peace with it. Some really loved making memory books for their friends and family. Some wanted nothing to do with anyone. My general persona with these guys is silly - I've found that get drawn into good humor and my willingness to act like a fool to get them to smile - but you have to go with what works for you. They can sniff out a fake immediately.
It's totally cliche, but when you're dealing with a sick kid in any role, you're really dealing with the entire family. I'm not sure what your role will be, but you may very well spend as much time with siblings, parents and other family as the kid. Siblings especially will be going through a difficult time they don't quite comprehend, so make sure you keep them included in things like crafts/play/etc, as long as it's appropriate. Depending on your volunteer role, they may get to know you quite well through the process.
Finally, be really cognizant of your attitude. Kids of all ages are wonderful at picking up on the emotions of others. If you're there to be a happy distraction, company or something else designed to be fun, make sure you can get there emotionally before going into the room. Like WheezyBaby said, that may involve debriefing before and after things, but when you're there, be as present and in the moment as the kids are. Bad things are going to happen, but you can help make that moment more fun.
I've found it to be a wonderful, painful, uplifting and at times terribly sad experience. I would not trade it for anything in the world.