Bbq desserts need to be simple I think. Can you take icecream wherever you're going? Or is that too complex?
A favourite with kids is to take a banana, split it lengthways with the skin still on then stuff the cavity with as much chocolate as you can (whatever chocolate you like frankly). Wrap in foil and toss into the coals after you've cooked the meat until the chocolate is melted and gooey.