Do you want them to close? Or are they just panels to hang on the sides? If you want them to close and if you want them to hang to the outside of the window so they leave the glass exposed when they are open, then those won't be wide enough.

For sources for curtains, look at overstock.com and jcpenney. They both have a huge selection. For me "kids" curtains need not be much different from regular curtains...it's about the color. You can always add the kid touch in other elements such as pillows and accessories and wall art throughout the room. This way the curtains will last even as the child ages.

They'll be skimpy looking at that width, but depends on the heading. If they're rodpockets, they may be OK, but grommets I'd say no.

You're also looking at a wider width. The rod should extend beyond the window on each side, so I'd suggest about 100". IMO, nothing looks better on the window than too skimpy.

You don't mention how high your walls are but 11" above is fine.

Thanks for the info. Yes, the curtains would close at night and during naps. The panels I found are 44" wide. So if I did four panels that would be 176" of fabric for 78" window. Is this too much?

The painters tape is about where rod would be hung. This would allow 96" long panels to hit about 1/2" from floor.

I would make the Rod extend a little bit further, look at Annie's picture, the rod's extend further. When the drapes are open you don't want them to block the light from the windows because the rod isn't long enough to push them all the way to the side.

I think the advice you're getting so far is good. Are you getting cotton panels that are washable? If so, I suggest you wash once before hanging to get any shrinkage out of the way first.