The Fisher F2 is pretty much the same as a Bounty Hunter Discovery 2200. The Fisher F4 is pretty much the same as a Bounty Hunter Discovery 3300 (The F4 comes with a DD coil, the 3300 comes with a concentric coil). The software in the Fishers are slightly more feature rich, but nothing dramatically different.
Both the 2200 and 3300 are great first detectors. Plan to keep them as spares after you upgrade. I have one of each and I use them to teach people how to metal detect with. I don't see why a kid would not be able to use one - they are very lightweight. I also have a Tesoro Compadre - it's much more sensitive to small gold chains and jewelry, but most people like the VDI display when they are starting out. Personally, I find that the VDI tends to make you skip over good targets that happen to be right next to bad targets (giving you a junk reading). Long term, you are better off listening for targets than consulting a VDI display, but that takes time to develop as a skill.
The low end Bounty Hunters are cheap toys but not worth much for Metal Detecting with - some of them couldn't find a metal car in parking lot. The midrange and above are fine.