Unless you're wedded to the idea of flowers in the center, would a container of colored Jello in the center work as a "lake" & maybe a fisherman figure on the side would do? With the right icing you could cover the edge of the container in the center.

Hope a good decorator will see this.

ginger's idea sounds fun.

Anything I can think of either seems sort of gross (like a big plastic fish coming from the center) or dangerous (like fish lures and hooks coming from the cake).

There are some "cute" kid fish cake ideas. But they are more along the lines of goldfish and Finding Nemo. Would those be of interest?

You can get some cute plastic fishing decorations, such as a small pole with attached line and fish, etc. at JoAnn Fabric. I have also seen the min. basket for fish. (can't think what the correct name is). The frosting could be a fluffy type with a bit of blue/green coloring and frost the top of the cake with "waves".