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We got my son a Razor electric when he was 8. It was perfect. The only “dirt” available was a parking lot.
They’re cheap, available, not too fast, don’t get stolen much. No starting required but you can secure them with a key. You can upgrade the batteries for lighter weight, more speed, longer range or all of above. Tons of parts on eBay.
We eventually got a CRF100F, but he was older and demonstrated interest. We had some great times.