Which backdrop should I get?

Don't limit yourself to thinking inside the box. Just because there are things called "backdrops" doesn't mean you have to get it. Depending on the size that you need you can spend $40 at a fabric store and use a couple of telescoping poles to hold it to the ceiling. Solid color sheets do in a pinch and can be had for a couple of dollar bills at a thrift store.

I will assume (and hope it doesnt make an *** out of me) that you don't have any off camera lighting. If you're using the onboard flash then I would go with darker colors to help reduce shadows.

Also don't forget that there are many many places out in nature that will make a great background for a portrait! Just think creatively!

You don't have to break the bank to get a good background for a photo. I am THE budget photographer here! lol

If you are wishing to focus on the portraits themselves, a mix of blue on gold mottled works fine, if you are looking to add background (say the Grand Canyon or the Pyramids) a flat solid light blue is the one.

Google the other info as for brands and cost as the price is reflective by the size and quality but if you are only doing this for simple personal use, stay cheap.

Almost anything will work for a backdrop. If the focal length is right it will be well out of focus anyway. Keep the subject about the same distance from the backdrop as the camera is from them. If you use any kind of post production software you can have any kind of background you want anyway.

In order to answer this properly I would have to know 1) what you plan to use for lighting,2) how many grand kids of what ages and 3) if you are looking for a modern or classic look. I would gladly respond to an email.

You might want to check into chroma key backdrops. With proper lighting, you can then easily replace the background in post processing.

White seamless paper. 9' roll = $60 or perhaps get 6 yards of 108" white muslin (wrinkles less than plain cloth, and that'll get you 8' for the wall, and 10' for the floor) = $20 with coupon from a fabric store.

This may sound ghetto, I use sheets. Just stich them together. white and black seems to work well. Other wise you should be able to do a search for photography supplies.