Does anyone collect old money (paper currency)??

I have three new stepnephews that I have not even met yet. They are 15, 14 & 10 so I since I didn't know what to get them for Christmas I bought them each an old uncirculated $2 bill. I was thinking of buying a simple frame and framing the bill and engraving each boys name on the photo matt, etc. so they can display it in their rooms. But this got me thinking, will this do harm to the paper currency?? Do you have a better idea of what I could buy or make as a display that won't cost much? Any advice would be greatly appreciated. Thanks,
Gemila