There's definitely a ton of youtube videos on that kind of thing.
Try for example
this channel for styles actually done on little girl hair. I bet they have some stuff with ribbons, too.
As an alternate idea for how to get it to show from the front, how about laying the braids around the head, crown-braid or milkmaid style in the end?
Something along the lines of what you see in
this video is a neat way to get them to stay there. And you could either braid the ribbon into the braids then or just use it for the actual taping itself, depending on what your nieces like better looks-wise :)
Then for tutorials on how to actually do interesting braiding techniques with ribbons, I rather like
this one, but you'd have to see how to make it visible from the front if the girls have too fine hair to start the braid with the ribbon at the front.
Mh, yeah, that's I think the best I can do off the top of my head, but if you need some more ideas I can definitely look into it more (but not right now since I should be studying).