Google "podsafe music"
If it's just for your family why worry about it being free? Just copy music off a CD you bought. The copynazis aren't going to bother you at grandma's house.
I made a disney video slideshow one year - and used music from the parks I got from frostwire.com. One of my favorites was the year 2000 parade music from epcot. I can't remember what it was called but it was an AWESOME parade. best daytime parade I think Ive seen at WDW