I would like to know what small children (1-2 years) dream, but we can of course never really know for sure. My daughter (2 years) definitely seems to have nightmares. She also seems to have conversations in her dreams, as she often speaks answers to questions while sleeping. Personally I very strongly remember some wild dreams when I was very young, probably around the age I started remembering things at all, so around 2,5 years. These dreams were often synesthesic, I could see sounds in extremely saturated, moving, three-dimensional colors, and could "taste" geometric forms like pyramids and cuboids. I have never taken LSD, but I imagine the experience to be quite similar. The closest (but not very close) thing I have seen to these dreams is the stargate sequence in 2001.