Lear is writing nonsense verse, so there's really no point in looking for a precise literal meaning. Nevertheless, one can at least hint at what he's driving at. The Dee is the name of several rivers that flow throughout various parts of the United Kingdom, and although Lear isn't literally referring to any of them here, it's more than likely that he had them in mind when composing "The Duck and the Kangaroo." Certainly from the context of the verse it would appear that he's referring to some kind of river where the duck would like the kangaroo to take him.

As for the "Jelly Bo Lee," well, that's a rather more difficult proposition. Lear may well have been influenced by Australian slang, as he was a great connoisseur of strange words. But one suspects that he simply wanted to conjure up a name that would evoke visions of far-flung, exotic places, the kind of place which the duck hopes to visit while hitching a ride on the back of the kangaroo.

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!