Just like with walls, kids just don't understand why everyone is always telling them not to touch windows. Why should they when it's so fun to press their little noses against windows, puff up their cheeks and happily bang on them with dirty hands?
Luckily for you, that old urban legend about washing windows with newspaper really works! A sheet of newspaper crumpled into a ball (don't try to do it with a flat sheet, or you won't get an even clean) combined with any glass cleaner or homemade ammonia-based cleanser will clean glass without leaving behind any streaks. Bonus: Newspaper will leave behind a dirt-resistant film that keeps glass from smudging.