1) People argue to go with softer reeds to make it easier for kids... it'll be 'hard' no matter what. Starting a reed that is closer in correct strength for the mouthpieces they have (mostly M13s) is better in the long run as long as you focus on some longtones on mouthpiece/barrel to begin. (Of course watching faces/embouchures as you do so!)
2) As a young sax player I started on a Rico 2. By Christmas my mom was still buying me 2s and my tone was a typical bad soft reed sound. It wasn't until someone finally got through to my mom (lesson teacher) to stop buying those. Your average parent doesn't think of going up sizes of reeds - they just buy what they've bought before. You may get stuck with kids on those reeds because it's what their parents buy.
3) Harder reeds are also slightly more resilient to chipping. They hold up better than the paper thing 2s.
We move onto 3.5's after they run through their first supply of reeds because that's ideal for their mouthpiece facings. That is the one thing you do always have to be aware of is which mouthpieces kids are on. On average a 3.5 tends to be good on most, but you may have some oddball facings sometimes.