I would question your parenting, frankly, if you're considering buying a desktop for a child's exclusive use.
I bought my daughter a cheap eMachines when she was 3. She had been watching Noggin on TV and kept wanting to go to Nogggin.com. She was not going to use my desktop or wife's laptop, so we got her a cheap PC to get online with. I put icons on desktop for her sites like Noggin, PBS Kids and Disney. She was navigating it like a pro within a week.
Now she is 11 and helps her teachers fix their computer issues at school. She also has a 13" MBP.
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