This is part of a poem. (that I happen to love). It's a very weird poem. In the course of the poem, a fish (the whiting) and a snail are talking. The fish wants to dance and is inviting the snail to come and dance with him (and the other sea creatures). However, in one of the parts of the dance, the creatures all get thrown into the ocean. The snail thinks that the dance is too dangerous, and says he's not going to come dancing. The part you quoted is the fish's response - he says that if they get thrown really far, it will still be ok. It will all work out in the end (they might be far from England, but they will be closer to France). "Turn not pale" means "don't be afraid." (In English, fear makes you pale.) And so he's saying "Don't be afraid, dance with us!"

The whole thing is quite poetic, not everyday English. "Then turn not pale" = Then don't turn pale, i.e. don't be scared "Come and join the dance" = I assume this means to join in with the adventure (or are they really dancing in the story?) Sorry, I am not familiar with Alice in Wonderland, but this is how I understand the language.