Here are the medals for this year's Dark Side races (the 5K medal and kids races medallion are pictured):
http://www.starwars.com/news/feel-the-power-of-rundisneys-2017-dark-side-medals-first-look
Next year's medals will likely be different, but this gives you an idea of what to expect. I would say that the 5K medals are up there with the longer race and challenge medals, but likely a bit smaller. The kids race medallions are a softer rubber material, normally a little more colorful than the other medals (I personally think that they always do a great job with the kids race medallions, but I have read that sometimes the kiddos are disappointed that it's not a "real" medal. YMMV on that one).
To expand on the stroller with a baby - as others have said, this is no longer allowed. Disney used to allow this years ago and then, I think, as the races grew, took the option away. You will understand why when you actually do the race - these races are crowded, and going through the parks means that there are portions with narrow pathways. I believe it is a course crowding and safety issue - safety of other runners having to dodge a stroller and safety of the kiddo in the stroller (because what if someone knocks into the stroller when you're running?).
For the 5K (as said above, 5 years and up), keep in mind that it starts VERY early. A 6AM start means you and kiddo will need to be at the starting area by 5AM. The kids races also have an age limit, in case that is something you end up doing. That is 8 years and under.