I have tried to do some research on this to help you out, but all I have been able to come up with are "possibilities". I am hoping that perhaps someone from Australia might be able to help you out because I believe these terms are "Aussie" slang.
This is one of Lear's seemingly child-like poems with a deeper meaning. The duck and the kangaroo are very different, but they resolve their differences by cooperating and achieving a mutual goal, which is to travel around the world.
As far as I can tell, the "Dee" may refer to Dee Why Beach, which is a long beach in Australia. Perhaps the author envisions the kangaroo and the duck flying over this beach as they go around the world three times. The "Jelly Bo Lee" was a little trickier to figure out, but it is most likely another geographical feature. Sometimes the word "Lee" is used to refer to a coastal feature, so perhaps it is The Leeuwin-Naturalist Ridge, a geological feature stretching from Cape Naturaliste to Cape Leeuwin. Maybe there are jellyfish along this stretch - "jellies". In any case, both of these terms refer to places that the duck and the kangaroo traveled over.
I hope someone else can give you some more specific information!