You can pick up the medical kind at some places don't confuse them with dust masks because they are very different. I like the fold down flat version vs the preformed because it packs very small.
As for NBC rated masks, if I were in the market for one I would be considering the M95 mask. You will pay a little more for it but unlike others it has a 20 year shelf life and its filters 10 years. Make sure you get the NEW full NBC filters for it. I suggest a minimum of 2 filters per mask.
http://www.approvedgasmasks.com/scott-m95.htmThere are other masks out there that will work as well but for an extra few $ a mask, you can have the best.
As for kids masks I really have no experience with them. There seems to be a lot to choose from for kids.
http://www.approvedgasmasks.com/child-masks.htm I would contact the vendor and ask LOTS of questions and ask about warranties and guarantees.
Don't forget to shop around and buy NEW only.
There are a few things you should never buy surplus or used. Underwear, parachutes and gas masks.
Mike