Turns out that these are called koinobori (鯉幟), which means "carp banner." Hmm, that makes sense. :)
In Japan, the carp ("koi") is a symbol of perseverance, strength and determination. So the carp flags represent advancement in life, as well as good fortune and luck. They're flown in honor of sons, in hopes that they will grow up healthy and strong.
Each member of a household with boys in it gets a flag. They're flown as a set over the roof of the house. Traditionally, black fish flags (the biggest) are for fathers, orange/red for mothers, blue/white for boys and red/pink for girls.
Flags and other Childrens' Day merchandise are sold at Universe.
Children's Day on May 5th is the last holiday of the three consecutive holidays that make up Golden Week. May 3rd is Constitution Memorial Day, and May 4th is Greenery Day or Nature Day.
Because there is no school (and in many cases, no work) on these holidays, Golden Week is a popular time to travel. That's how I'm able to be in the Philippines next week! (By the way, I just wrote a new post in "Keep Shining.") So this will be my last post for a little while since I'll be traveling.