Yay! Welcome to the disboards! Sometimes there are charities and travel agents that have bibs still available after the sellout. 10Ks sell out the quickest, since some of those are put together with people running the challenges (10K and half), so that leaves that many less 10K "only". One thing with using a
travel agent, per some newish rules, they have to book you on a room and park ticket package along with your bib(s). I am not 100% on details, I know some people on this awesome thread can give you more info
I'm running the 5K w/ my oldest (DS11), we've done a few together but this is the first time without the rest of his siblings and DH. It's a lot of fun and especially w/ the 5K you'll see quite a few kids with parents, the runDisney 5Ks seem to be more "family fun runs" anymore.