My DD actually had an Etsy store for a brief time to sell her art (all her idea) when she was about 6. That would be the easiest way and there are other children with stores there. However, the reason we closed ours was that we were getting some odd interest, including someone who just wanted to send DD money and to "commission" works of angels and stuff. I think she (of course, was it a she?) was harmless, but it spooked me a little. It's a risk.