I am delighted. I was able to find a visual language editor that is freely customizable. Please tell me, I would like to change the UI of snap to be child-friendly so that children can learn the basics of programming using snap. Is it possible to change the colors of the screen and the arrangement of the UI?
Currently the only way to change the colorbtheme is by using the flat design. It gives the whole editor a light theme. There is no way to move things around. If you're ok with editing the souce code, then you should look at this file
tl;dr: Yes.
Since the source code is freely available (AGPL), you can change it however you like! To find the source code, click on the Snap! logo 
in the top left corner of the Snap! window and choose "download source" from the menu. (It takes you to a Github repo.)
Certain changes are accessible within Snap!. For example, you might be interested in "Zoom blocks" in the Settings menu (the gear icon). You can create new block categories; you can hide blocks; you can provide translations of your custom blocks that are integrated into the (human) language translation feature. There is one hideous alternative skin (Settings>"Flat design") that I fear you might like. Check out the Reference Manual for all the gory details.
We might have differences of opinion about what is or isn't "child friendly," but that's okay, that's the whole idea of free software.
A quick note about the first link...
The mod is incomplete, and doesn't include all of the blocks in the current snap editor.
Yeah but it’s still pretty good 