I think most healthy active kids can probably get round 5k if the pace is right, but as has been said, the majority of kids tend to be all or nothing when the gun goes when they first start running properly. If you can reign her back for the first few minutes then she'll probably get round fine. Don't be afraid to run/walk if needed, but getting them to the finish line I think is important as it gives them that sense of achievement and they get to receive a finish disk and get scanned in, and then see their name in the results. That makes them feel like they've won something and also that they're part of something, and that makes them more likely to want to do it again.