Over all is pretty nice, one thing that you could do to greatly improve would be to get rid of white dots on the inner edges of the line art (the only fix to this that I could find was to copy and paste frames onto them selves. There may be a tool setting somewhere but I do not know flipaclip well enough to be able to give you a permanent solution).

They are so cute!! I love it.

This is fucking awesome

Spectacular

One problem that jumps out at me are the inconsistencies of the limbs. Lots of the legs and arms seem to very in thickness and length even on the same character.

One other thing is that most of the male characters seem to have the same or similar expressions. The facial expressions that your characters have says a lot about their personality's so if they all have they same expressions then they could come off as bland (also keep in mind that since this is the front cover you want it to be as eye catching as possible, so you would want the characters to look more interesting).

Now with the pink haired character I would recommend changing the way you draw her breasts. From this perspective it might not seem like a problem but it would be hard to draw them that way consistently from other perspectives. A great way to find alternative ways to do this without looking up weird things or having to look at anatomy would be to just tune into the Disney channel or cartoon network and just see how the different cartoons draw breasts and try to replicate that.

Now i would like to say that the character with the blue hair and the hoodie would have to be my favorite here. I think that the orange and blue sleeves matching with the colors of his power is a very nice touch.

With all that being said i would encourage you to continue this project. You seem to have a lot of potential and if you keep at it you will be making stunning works of art in no time. (And also never get rid of this or any art you make. Its always nice to see how you improve over the years).

Your art is soooo good, it all puts a smile on my face.