I reckon the perfect first Mac for a kid is the 13" Macbook, the one with polycarbonate body. It is very robust and forgiving to bumps and hits. The metal ones are a bit precious and dent with rough use, I'd only get them for older kids (13+ or so).
I'd also get it from Apple (online or retail store) rather than JB, even though it's $50 dearer. The one on sale at JB is the previous (superseded) model.
Oh, and yes, 13" works well with Snow Leopard, even though I'm a huge screen junkie myself (17" MBP and 30" ACD
)
Cheers
Steffen.