If they have ethernet upstairs then i would suggest getting a second router and give it the same name(SSID) and encryption password. Then they will appear as the same network to devices and they will connect to whichever is best.
You can use an old router if you have one as a wi-fi booster,this is a bit more involved and for more advanced users but there are some good tutorials on how to do it that anybody can follow.Here is one form lifehacker: http://lifehacker.com/5563196/turn-your-old-router-into-a-range+boosting-wi+fi-repeater
Here is a very good youtube video on using an old router as a wi-fi booster
You can also use an old router as a wireless access point which is much simpler. Here is a good tutorial: http://www.tested.com/tech/298-how-to-use-an-old-router-to-expand-your-wi-fi-network/
I keep my old routers just for this reason.I have used both of these methods for myself,friends and family.The latter method being much easier and less time consuming.