Since she has loafers in her school uniform, I assume you mean the frog outfit in the middle there?
My first thought is to build the bootcovers with the green parts treated almost like plush, but out of a suitable material for shoes. You would create pockets of space in the bootcovers out of the green fabric, and then stuff the area around the shoe so that they have the rounded shape.
You can also try attaching something round (pingpong balls might be the right size) to the base shoe, and then patterning the shoecovers around them.
I would recommend using the saran wrap or duct tape method of patterning for this, since the shapes are complex.
If you wanted something that wouldn’t involve sewing, you could cover your round objects in a stretch fabric and then glue them to the outside and top of the boot, or build the entire shape out of carved foam.
I hope that helps! Good luck :]