Either of the knives you have mentioned would be perfect. I lean more towards a folding blade as it makes it easier to carry and use rather than toting around a fixed blade everywhere on your belt(if they even have a sufficient way of carrying a fixed blade). You can't go wrong with a Victorinox by any means. The Farmer in itself is a wonderful knife, but if you wanted something with more tools(which young boys will insist they'll use) you can't go wrong with an Explorer, Huntsman, or Fieldmaster in that price range.
If you just want a good all around cutting tool without all of the tools, you can't go wrong with Kershaw or Spyderco or Spydercos Byrd line.
-Spyderco has the Tenacious line - Ambitious, Persistence, Tenacious, and Resilience(The Resilience is huge and not under $30). If you go with that line, since you're dealing with children the Ambitious or Persistence may be the best bet.
-Spydercos Byrd line has several knives that are designed based on a lot of Spyderco designs only they have a cheaper blade steel and are made in china. The 8cr13mov steel holds up well though.
Kershaw has many products for $30 and under. Some are chinese, others aren't.
You really can't go wrong no matter which way you choose to go. I just thought maybe a folder would be cool because boys love their pocket knives lol.
Good luck on your hunt.