I like the idea of this illustration. A hippo, care free and soaring through the air. This wimpy little turtle thinking "Oh God, what did I get myself into." It's a great concept and idea, and I think the coloring was really nice with great texture to the piece. Just some things I think might add to your piece and give more strength.
1. I see that the big top tent is at it's highest point here. There would be a main support beam to prop that tent up, but it cuts through your illustration and it might look like these two are going to crash into it. So I really don't know if you add it to the illustration or not. It's just a nit picky thing I noticed.
2. A hippo is a huge mass. Very large, very weighty. It needs some better construction to really convey that and make us (the viewer) believe that this large oaf can sail through the air with the greatest of ease. I like to see where the spinal column is in all the characters I draw.
3. This arm just seems out of place. She needs to have both arms extended if that turtle is going to stand a chance to catch her.
4. Know where the shoulder blades are in your character. draw through your shapes and really understand where all those forms connect.
Just to show you more about what I'm talking about I drew over your original drawing in Sketchbook Pro.
1. I use your same basic shapes, but I have to know where that spine is.
2. To understand the mass of a character or the volume I will draw circles as if it was a 3d model. This keeps the character from looking flat later.
3. Need to know the distance of the hips to one another. The circles give me an idea of this and if I draw lines from one hip joint to the next it will keep it honest as to where it need to be placed. These are all important for readability and selling this far out idea as believable.
Just another example of needing those hips and shoulder to line up....always draw through your shapes.
1. Understand your sense of motion...everything should follow that one fluid motion. See how the ruffles in the outfit all flow back as she flies through the air. The wind pushes it back.
2. Now with that said the tail would more than likely be flowing backwards, but to add excitement and break the fluid motion I drew it forward. After all illustration can and should stretch the truth somewhat.
Lastly I think the trapeze needs to have some motion as all of her weight releases the trapeze shoots or springs away from her.
I hope this helps somewhat. I love drawing cartoon animals so I really felt like this was one are where I might really be able to help someone out with.